Well let's look through each island's Pokedex and list out the Pokemon you can get, not counting repeats (and assuming you don't go back to a previous island):
Melemele: Starters, Pikipek, Yungoos, Rattata, Caterpie, Ledyba, Spinarak, Pikachu, Grubbin, Bonsly, Happiny, Munchlax, Slowpoke, Wingull, Abra, Meowth, Magnemite, Grimer, Growlithe, Drowzee, Makuhita, Smeargle, Crabrawler, Gastly, Drifblim, Misdreavus, Zubat, Diglett, Rufflet, Vullaby, Mankey, Delibird, Oricorio, Cutiefly, Petilil, Cottonee, Psyduck, Machop, Roggenrola, Carbink, Sableye, Rockruff, Spinda, Shellder, Bagon, (via Island Scan: Totodile, Deino, Horsea, Klink, Chikorita, Litwick, Cyndaquil)
Okay, not bad. Actually quite packed as said. You can form a pretty solid team with this. There's even a "replacement" or the Starter types you didn't pick so you can have Grass, Fire, & Water on your team (not counting the Island Scan which can give you one of the Johto Starters, not to mention Deino, Klink, and Litwick). Then we come to some powerful or useful ones like Munchlax, Abra, Magnemite, Makuhita/Machop, Gastly, Zubat, Diglett, Rufflet/Orcicorio, Cutiefly, and Rockruff to name a few that jump out at me. So if you try to catch as many Pokemon you can find before moving to the next area you can form a pretty solid team from island one. The Pokemon Colosseum games didn't even offer such a wide variety throughout its whole game!
Akala: Lillipup, Eevee, Mudbray, Jigglypuff, Tauros, Miltank, Surskit, Dewpider, Fomantis, Morelull, Paras, Poliwag, Goldeen, Magikarp, Barboach, Tentacool, Finneon, Wishiwashi, Luvdisc, Corsola, Mareanie, Feebas, Alolmomola, Fletchling, Salandit, Cubone, Kangaskhan, Magby, Stufful, Bounsweet, Comfey, Pinsir, Oranguru, Passimian, Goomy, Castform, Wimpod, Staryu, Sandygast, Cranidos, Shieldon, Archen, Tirtouga, Phantump, Nosepass, Pyukumuku, Chinchou (via Island Scan: Spheal, Luxio, Honedge, Venipede, Bellsprout, Marill, Gothita)
Alright, if you don't have a full team leaving Melemele you should after Akala. We've got some more Grass, Fire, & Water in case the ones one Melemele weren't doing it for you (especially for Grass and Water, you may want to consider replacing the one you have if it's not the Starter). We also got Eevee in case you really needed a certain type covered. Of course we have even more of the advertised new Pokemon so you gotta try those out. There's even a lot of Pokemon who have a gimmick or have a battling role giving them some purpose. All in all, Akala is essentially the finishing touches island.
Ula'ula: Trubbish, Skarmory, Ditto, Clefairy, Minior, Beldum, Pancham, Komala, Torkoal, Turtonator, Togedemaru, Electabuzz, Geodude, Sandile, Trapinch, Gible, Klefki, Mimikyu, Bruxish, Drampa, Absol, Snorunt, Sneasel, Sandshrew, Vulpix, Vanillite, (via Island Scan: Swinub, Duosion, Roselia, Staravia, Vigoroth, Axew, Rhyhorn), Zygarde (10%, though you'd probably be able to get to 50% before the Pokemon League)
Now, there's some good Pokemon here, but the question is if it's too late? Like the only reason I can see you replacing a team member is if you specifically wanted one of these Pokemon (or an Ice-type). But by now you probably have a strong team so would you want to start over with a Skarmory or grind with the Beldum? Heck, for some I would say you're a little bit too late like with the Fire- and Electric-types. Not even the Island Scan Pokemon are that great. And really, Ditto? So pretty much just a specialty island, you only replace a Pokemon if you find a Pokemon here you REALLY want to use (probably Mimikyu or Minior). Oh, and you also can get Zygarde 10% if you roll like that.
Poni: Wailmer, Dhelmise, Aerodactyl, Exeggcute, Gastrodon, Lapras, Granbull, Sharpedo, Murkrow, Jangmo-o, Dratini, Solgaleo, Lunala (via Island Scan: Serperior, Samurott, Emboar)
... yeah, unless you wanted to train up a Jangmo-o (or Dratini) now all these are just dex filler (though that free Aerodactyl is nice, and of course the Mascot Legendary if you just want to get through the League with little problem). Since Poni Island was the last island maybe they could have added more of the Pokemon without an evolution family (that's worth wild. Dhelmise, here, really?) or some with maybe a competitive use like a good wall (I guess Gastrodon is like that, but I'm thinking along the lines of Ferrothorn or Bronzong).
Post Game: Island Deities, Ultra Beasts
Melemele: Necrozma
Akala: Porygon
Ula'ula: Cosmog
Aether Paradise: Type: Null
Poni: Emolga, Scyther, Riolu (via Island Scan: Conkeldurr, Togekiss, Leavanny, Eelektross)
Aside from the Legendaries, Island Deities, Ultra Beasts, and Type: Null why keep any of those other post game? Like Porygon could have been fun to use (if you had a way to evolve it) and Scyther & Riolu might have been able to make some players choose to replace a Pokemon in their part for.
Hmm, you know, after looking through this I got a new idea for a playthrough challenge: Island Native Challenge. You can only use the Pokemon native to that island, so each time you get to a new island you need to box all your old mons and start over (after using one to catch the "starter" for that island, of course).