Here's an unpopular opinion: Unova's circular, railroading region design isn't that bad to me, especially compared to other games. This is going to be a long post, and it will cover all the times you can go off the beaten path in all the main games save for the postgame (in every game save briefly Johto) which in almost all games are pretty plotless save ORAS and USUM, and by definition optional if you just want to beat the games and be done with them.
Let's step back and look at Kanto. You don't start getting branching paths until about Cerulean City, and since both paths (Gym, Route 24/25) are required (though you can come back to Misty later after S.S. Anne) the optional areas don't start coming into play until Diglett's Cave and Route 11 to the west of Vermillion. From when you receive Cut on S.S. Anne, you can sequence break, theoretically tackling the next five gyms in any order save for Koga to get to Blaine. But how many players actually go out of their way to do this when their Pokemon would likely be underleveled? You also have the branching paths to Fuschia via Cycling Road and Routes 12-15 (the latter of which are quite an insanely long trek) as well as some of the water Routes around Fuschia/Cinnabar, Seafoam Islands, and the Power Plant. Overall, this game has a lot of areas that let you go off the beaten path, which is impressive for the first games in a franchise. However, this design would come at a cost soon...
...which is the cost of Johto having a pretty atrocious level curve post-Morty. Prior to Morty, the area design was mostly a straight path (you can't do much in Dark Cave until you get Surf and you need Surf and to a lesser extent Strength to navigate most of the Ruins of Alph save the "Upper-right entrance 1" which requires no HMs). Once you get Surf, the region lets you challenge the next three gyms in any order as well as explore some earlier areas with it to find secrets (Dark Cave, Ruins of Alph, Union Cave, Slowpoke Well, Soft Sand on Route 34 from Cooltrainer group, Mystic Water near Cherrygrove). Remember that you don't have Fly yet until beating Chuck, though, so you do have to do a fair amount of walking even though you've dealt with Sudowoodo. This broad region design kills the mook level curve (while the routes to Olivine and Cianwood are okay with decently powered mooks, anything around Mahogany is pretty laughable, with most trainers sporting stuff roughly 10 levels below Pryce and giving pitiful experience as most are unevolved). You can also explore some areas with Waterfall much the same way after Clair (Mt. Mortar and Route 47 come to mind). While it may not seem that bad on paper, you will be underleveled for the Champion even if you do these (I tackled some of the off-the-beaten-path stuff once and I was still underleveled come the League). Fun fact: did you know there are trainers at the Lake of Rage post-Team Rocket HQ? They help with grinding but there isn't much reason to go back that way unless you know TM36 Sludge Bomb is on the route prior. Post-game, Kanto is much the same. Even if you tackle all of Kanto's trainers you will barely be on par with Blue's levels and Red is Red: have fun grinding for him. Overall, it was a nice risk, but one I'm glad they dropped to make the general difficulty smooth instead of arduous. Thankfully, later gens actually had much better level curves and will be more concise design-wise.
Next up is Hoenn. Upon getting to Dewford Town, in the originals, you can sequence break to Slateport City and leave Brawly for later upon delivering the Letter to Steven. You can also do the same for Winona at Fortree City, but this and the Brawly skip were blocked off by NPCs in the remakes (and you have to beat Brawly pre-Norman anyway). Back on topic, Slateport and Mauville have branching paths around them (Route 103, Trick House for the former, Route 117 and 118 for the latter, with Cycling Road being in-between both once you get the Bike in Mauville, which you can also use to fully explore Granite Cave. From there, the game more or less is linear (save backtracking to Mt. Chimney) until post-Flannery (Route 111 aka the desert) and especially post-Norman when you get Surf. With Surf, you have many optional areas: New Mauville (original, you can only really explore there post 8 badges in ORAS), the various water Routes between Petalburg and Slateport (105-109), Route 115 near Rustboro and Abandoned Ship/Sea Mauville (fully explorable once you get Dive). The game then mostly settles onto a straight path until post-Winona, with the indoor part of Mt. Pyre being optional and Route 123 below it being completely unnecessary), Once you get to Lilycove and beat the evil team Hideout, Shoal Cave as well as so many routes with Surf open up I'm not even going to try to list them. After you get Dive, you can also go after the Regis (but good luck finding them the first time through or having a Wailord/Relicanth) as well as explore Abandoned Ship/Sea Mauville fully. Once you get Waterfall from Juan/Wallace, the only real optional path to explore with it to my knowledge is part of Meteor Falls, and then comes endgame afterward. Overall, many optional areas that don't kill the overall level curve, but I think most first-time players would get fatigued by the infamous amount of water (insert meme here).
Fourth is Sinnoh. Hoo boy. Save for Oreburgh Gate with Rock Smash, the optional areas don't start happening until about Eterna City, with part of Mt. Coronet and one route near Eterna being available (as well as the Undergound if you count that). Following Hearthome, you have the Lost Tower (which isn't optional in DP due to HM04 Strength being on the top floor instead of at the start of Iron Island) and the Solaceon Ruins. You have two ways to get to Pastoria (being very similar to the midgame trek to Fuschia in Kanto) with the Ruin Maniac Cave and the (Sinnoh) Pokemon Mansion being small distractions. Once you get Surf at Celestic Town, the non-linear region opens up a lot: you can explore Ravaged Path for TM03 Water Pulse, Oreburgh Gate for slight experience/items, Fuego Ironworks for TM35 Flamethrower and a few new mons like Magnemite/Magmar, part of Floroma Meadow for some good items, behind Valley Windworks for TM24 Thunderbolt, part of Mt. Coronet's first floor, the water sections of Valor Lakefront AND the water sections below Sandgem Town. You can also get minor items like TM19 Giga Drain from Route 209 and Mystic Water in Pastoria City as well as other areas. Seriously, those are a lot of areas; chuggaaconroy spent roughly 45 minutes exploring them over two separate videos. After you finally Surf to Canalave, Iron Island is completely optional save for getting HM04 Strength from Riley at the entrance in Platinum, but there is a not-insignificant amount of mooks there providing helpful EXP. After you deal with Team Galactic (getting TM38 Fire Blast at Lake Verity after doing so), you trek to Snowpoint and beat Candice. Now that you have Rock Climb, you can trigger the Galactic gauntlet by heading to Lake Acuity stopping to get TM13 Blizzard in the process, and you can also fight a few mooks on Route 216's Rock Climb portion to get TM13 Ice Beam and TM29 Psychic near Celestic Town, which ends pretty much all of the optional areas. Honestly, this is a great dearth of areas and a great way to handle backtracking despite this generation's infamous amount of HMs. It also doesn't ruin the level curve, though it's still a problem endgame (hello level jump from Volkner to Cynthia)
And we are now at Unova. In BW1, you have the craziness of Castelia City which is anything but linear if you want good items and EXP from the Battle Company. You can argue Desert Resort and Relic Castle aren't optional because you need to pass through them once for plot later on, but they still mostly are because mook levels there are scaled to Elesa rather than post-Brycen (save the Plasma Grunts in Relic Castle). Route 16 and Lostlorn Forest are optional but have good EXP. As for Surf exploration, you can tackle Routes 17 and 18 (where TM24 Thunderbolt is located), Mistralton Cave (one of the best in-game mons in Axew, Rock Slide and Coballion are there so it is well worth your time, and Coballion also unlocks the deeper part of Pinwheel Forest including Rumination Field where you can catch Verizion), some minor items near Driftveil City, Striaton City and Unova Route 3, as well the rest of Wellspring Cave (has a few high-leveled mooks and TM47 Low Sweep). Aside from this, the only other optional areas are Moor of Icirrus (lol) and Shopping Mall Nine. So, not as many as previous generations, but it doesn't hinder the level curve. Sure, it's a circle and some like exploring, but bear in-mind this still accounts for about an hour and a half of exploring (from chuggaaconroy's BW series this took up four lengthy videos). After the huge amount of optional areas in previous games, it may be nice to some to have a game they can mostly speed through, with rewards for those who go out of their way.
Why do I defend Unova's circular design? Because of BW2, goodness gracious. Once you get to Castelia City, wow. Battle Company, Castelia Sewers (some parts of which can only be explored in certain seasons), part of Relic Passage, Castelia Gardens (though small) and the city itself. Post-Castelia, you get Desert Resort (truly optional now albeit helpful), part of Relic Castle as well as Route 16 and Lostlorn Forest again. Then comes post Mine-Badge, with the other big part of Relic Passage and by extension part of Relic Castle (with a level 35 Volcarona you can make usable by tutoring Signal Beam in Driftveil) open up. Next, you get Surf, letting you explore Castelia Sewers fully in Spring and Summer (I think), Mistralton Cave again (no Coballion...yet), some minor items and mooks around Flocessy Town and an item in Virbank City, as well as minor items around Driftveil again. You can argue Celestial Tower is more or less optional as you only need to grab the Lucky Egg on the first floor to progress plot, with the higher floors providing good EXP. Strange House is optional but helps if you want a Chandelure as the Dusk Stone is found there. Route 14 and Undella Bay are completely optional but again have good EXP. Even on the main path, you can skip a lot of mooks on the way to Opelucid with careful movement, though the infamous Gym drought is still a little fatiguing. Near Opelucid, you have Route 9 and Shopping Mall Nine. On Route 23 with Cut, you can access a lot of optional trainers and items (this is how I believe weaksauce HMs like Cut should be done, rewarding out-of-the-way exploration but not requiring it). Once you get Waterfall, you can use it soon, with the optional Abundant Shrine on Route 14 and quite a bit of Victory Road needing it if you want optional EXP and items. In fact, this is my favorite Victory Road due to all the optional areas: lots of EXP, tough trainers, and a long but fair gauntlet. You can also backtrack to Route 11 for some optional stuff if you choose. In short, these games have optional paths everywhere starting from Castelia, but in my opinion they are well-handled.
And now we have XY, where the circular design caught up with them. Man, they really cut down on the optional areas here, even compared to BW1. In their place, they have a lot of area diversions before you can continue trekking Kalos (Parfum Palace, Glittering Cave, Kalos Power Plant, Frost Cavern, standard Lysandre Labs/Team Flare HQ stuff, Route 20/Winding Woods). The only optional areas are Azure Bay, Lost Hotel, Terminus Cave and Route 16, save a few areas where you can get optional items with HMs like near the Badge Check Gates (though not nearly as many as previous generations). I guess you could count most of Lumiose City optional, but good luck navigating it without a taxi.
And finally we have SM and USUM. You'd think having four islands would shake it up...but sadly, you are railroaded even more than usual (though I like the plot it's boring on repeat playthroughs). There are some optional areas like the water routes and areas near Hau'oli City, Route 7, Hano Beach and Thirfty Megamart, Seaward Cave, Kala'e Bay, Haina Desert, Ten Carat Hill, Hau'oli Cemetery, that power plant near Malie City and Blush Mountain (the last two are short though) but otherwise there's not really any alternate paths, and some of them (especially the first three) are really easy to miss your first time unless you like filling out your map. Even though that list seems like a lot, most of those can be completed in five minutes (save Haina Desert and maybe Seaward Cave/Kala'e Bay). On the other hand, the main routes are densely packed and the difficulty is well-balanced, so it evens out a bit in my eyes.
Thanks for reading this massive wall of text. I know this got a bit tangent-y, but I hope you enjoyed reading all of it. The circle design in Unova was mostly fine in my eyes save for Kalos where it really felt like you could almost never go off of the beaten path save for plot areas. Personally, I think BW2 Unova did non-linearity best, though Platinum comes a close second with all that post-Surf stuff. What do you all think?
