I've recently finished both a nuzlocke in Renegade Platinum and a normal run in Platinum (in a 2 days marathon, back to back), and having this gen more fresh than ever, I've to say that the original DPPl is by far the hardest official generation in all of Pokémon (only founds competition in BW2 C. Mode). The levels on Pokémon trainers are high af, and if you are training a full team with no grinding on wild mons, you'll always, always be underleveled against special trainers. I even wiped 1 time against Gardenia Roserade because I didn't expects its damage, I opt to not full heal Staravia and it did like 35% of my remaining health with a non effective Mega Drain. After that I couldn't do anything at all. But anyway, mixing my past experiences in both normal runs and DPPl competitive with my recent runs and inspired by Suspicious I'd like to also share some opinions on certain mons i think i will use on bp.
Here's the thing. When I first played Pokémon Diamond, I choosed Turtwig. Surprisingly, Torterra is the only Grass/Ground Pokémon in existence. It learns some powerful moves like Crunch, EQ, Wood Hammer, Stone Edge, can setup with Curse, has Synthesis, access to SR (altho all starters share this), and enough defenses to take anything and relatiate. Empoleon is also a good mon, water spam is like fairy or ghost spam, rekts everything, water/steel is a solid typing, it's slow but not slow enough to get outspeed by common mons -and can run agility, flashbacks of lg agility sub petaya empoleons- and we all just know how good water + ice is by itself. But...
Design wise, Infernape is the one I like the least, but it's probably the most fun to use. A fast mixed attacker with everything in its movepool, the ape can act as a lead with Fake Out + SR plus 2 stab moves, it learns CC, Flare Blitz, Fire Punch,
U-turn, Stone Edge, tpunch, SD, Bulk Up AND Nasty Plot (???), access to both Vaccuum Wave and Match Punch, Focus Miss, Grass Knot... No, seriously, the only reason why this mon fell after IV Gen, amongst other gods like Cune etc, is because of the insane powercreep, because this thing is a fkn monster. It also rocks very respectable attack and spa and a great speed tier. Oh, I forgot about Taunt, this guy can shut down other mons who attempt to set hazards or boost, it even gets obscur shit like Endeavor, cmon... and you can run it with cb, specs, lo, sash, it doesn't matter.
Now, there's something that always happens to me when I play DPPl, and that's that Starly becomes my starter. Take quick access to stab tackle/quick attack + wing attack and add some fast Intimidate when it evolves into Staravia, meaning it becomes an utility mon, a high base attack, decent speed, you get return at the start of the game... I also cant get over the fact that it lears Close Combat. Today it may sound like something normal for young players who have grown with BW/XY, but for a Flying/Normal mon to get this coverage was absurd, in my mind, it was like a Pidgeot or Fearow who could destroy Rock and Steel types should they swich into Brave Bird. Wtf was that. It has decent enough attack paired with extremely high bp-perfect accuracy moves (brave bird, return, double edge), gets u-turn, defog, and if it happens to be Reckless instead of Intimidate then he is claiming some souls before dying.
Luxray is not a bad mon by any means and I usually get it for double intimidate, but i think i'll pass this time. While it's not a bad mon at all, I feel like in the long run it doesn't add much to the team, and whatever it does can be done even better with an offensive grass type (/ gyarados, if the games were more difficult gyara could be a problem in a team with no electric attacks, but that's unfortunately not the case...). It's learnset isn't the best also, it basically gets electric stab + crunch while leveling up, and you only get a noticeable spammable move (Discharge) at 56/64. It just doesn't have much options since the elemental fangs are obtainable via egg moves and not lvl 1 relearner.
... I forbade myself to use Gyarados ever again in a normal playthrough of Pokémon (even some lockes). I don't think there's a more OP mon in the history of the series since generation IV. Water/Flying in an extremely, extreeeeemely difficult type combination to deal with. x4 electric weakness means nothing when they'll be realisticaly facing a +1/+1 Gyara all the time, because Water/Flying with intimidate can set up in the face of a lot of things. Bulky as fck, fighting resist, DD at lvl 44/36, Ice Fang to outspeed and kill any dragon (fe: garchomp), STAB waterfall (which can flinchax you), after +1 attack it becomes a pseudo dark type offensive mon with Crunch, stab Bounce (paired with waterfall for possible paraflinch scenarios), gets taunt, have very good synergy with u-turn ape (i play on set switch ins) since it's inmune to ground and resists water, it also is one of my favourite mons evers design wise. But seriously, Gyara ingame makes every other mon pale. You'd think Garchomp is broken in gen 4 -ingame- and it is, but at least it's still susceptible to getting outspeeded by Weavile, fast dragons (latitwins), and ice shard mamos etc; gyara rocks leftys + intimidate and after a boost no living thing can seriously threat it in DPPl (again, ingame).
I love Hippowdon (female version is even better). Instant recovery option in slack off, stab eq, toxic user, gives me a hard rock resist in a team that is specially weak to rock spam otherwise, sand stream to chip away mons, yawn, gets the fangs by move relearner, switch ins in everything and tears it down slowly.
Another option would be
A somewhat mixed defensive variant with roost. It's esentially a worse version of Garchomp, but I don't like Garchomp in my team, that thing is too much. Flygon also provides me with rock resist and another u-turn user, and can put a bit more pressure offensively without losing all the momentum.
Bronzong is criminally underrated in DPPl runs. Hard resist to psychic and inmunity to poison (there's no corrosion here)/groudon, only 1 weakness (altho it's been nerfed due to steel resists change, still solid), hits hard enough, can annoy whatever you sent to it with confuse ray + hypnosis (i don't like it competitively but i value this a lot ingame because it helps you with catching wild mons), learns psychic, extransensory, eq, iron head (which is now more valuable thanks to the introduction of the fairy type), explosion, can set up screens, gyro ball, learns trick room... Also another rock resist + a fairy resist won't hurt.
https://www.smogon.com/forums//media/minisprites/porygon_2.png
I value a lot hard counters to ghost spam. Porygon2 happens to be one of my favourite mons too and you get both it and upgrade relatively quickly in Platinum (i hope it is the same in DP remakes). /fire, it learns everything. Gets bolt beam coverage, gives me an inmunity to ghost, a way to shut down quickly opposing gyara thanks to trace, pseudo resist to dark spam (team is so weak to weavile, only ape resists it and its not reliable since it lacks proper bulk + slack off ingame), recover, and can get with a lot of things and even end up sweeping with charge beam.
So i think the team could be something like that. A good team by ingame standards, without abussing too much broken mons. I dont care about waters that much because ingame there they are just fodder for dd gyara to set up. I can always change Bronzong for Zone, which gives me an inmediate offense against waters, preserves most of it traits and has more offensive pressure on the ofense.