A hack of Platinum? Hmmmmmm... well, for one I'd like to see the good quality-of-life HG/SS things. That means your Pok?mon following you and having reactions to the environment. There, I said it. That's one thing that none of the other games ever got right.
I'm not really a fan of tryhard competitive romhacks (come on, who the hraka puts a type-covering Smogon rank 6-mon competitive team
on the first Gym Leader? Even with a modified wild mon distribution, at that point the player is limited to $NATIONAL_BIRD and other such cannon fodder) so I definitively do not want to see the difficulty amped up
that much; but I do want to see some sort of difficulty spike that becomes more pronounced as you explore the game more, without losing balance. Raising the difficulty somewhere around 4th/5th gym or at the E4 at most makes more sense, since by that time you have access to much better catch distribution and access to items that actually help, like TMs.
Unless it is for plot reasons, in any ROMhack I expect to be able to at least catch all the normal, non-Legendary Pok?mon available in the NDex up to that gen, so if not a 493 at least a 470something in the wild (minus Starters, minus some Legendaries, minus fossils, stuff like that) kind of game.
At this point, I guess backporting some of the mechanical foundations from G5 onwards, such as upgrading the type chart with Fairy Type and , should be a given. However I don't think those could be considered an actual "requirement" considering the difficult job that it can be (adding the missing 300-something Pok?mon?) or that some things might not be able to be ported without significant changes (such as some Abilities perhaps). But at least having some things from further generations would be nice.
Other than that, it being a Platinum hack I'd like to see some things, though I do not expect them or require them, they're just niceties to have.
- Improve the saving time.
- Easier access to TMs. If they are not made reusable like in G5 onwards, at least there should be a store where one could re-purchase / re-earn all TMs at some point before the Champion fight.
- Some sort of leveling / balance system such as equilevel (where the level of the wild Pok?mon increases with your own, up to some threshold).
- More double battles. Making some boss battles be double battles as well.
- Postgame should have some actual plot rather than merely grinding in the competitive facility du jour. Perhaps grant access to new areas in some routes, or change some towns or routes as a reflection of how time has passed.
- A sidequest at some point during the game that allows you to obtain one (or more?) of the starters from another generation.
- Rework move tutors so that they are part of sidequests as well.
- Rework some of the event stuff to be made into a plot or sidequest inside the game, for example the Azure Flute.
- A sidequest that gets you access to a new location in Sinnoh that is not in the games, such as how the anime has Sinnoh-specific towns or the location of the movies (Alamos Town, Michina or something like that).
- Basically, sidequests! Having more stuff to do with the game than the GF standard "be handholded through the plot".
- Even if access to some Pok?mon is blocked until the postgame, evolutions should not be. That means that if Eevee is available in the first run, all of Eevee's evolutions should be reachable before the postgame.
- Full body back sprites for Pok?mon.
- Music! One of the coolest things in Pok?mon, it would be nice to see improved remixes of the tracks or completely new tracks allright.
- Faster in-battle notification engine. One of the things Pok?mon still can't get right and ROMhacks can't seem to either is the problem of using a Poison attack on a foe, then a two seconds pause, having a message "The opponent is poisoned!" then waiting two more seconds then having the [PSN] icon appear on the foe then two more seconds of pause then the enemy wiggle, then two more seconds and have the enemy HP bar deplete.
- Updating part of the game engine to put some of the status effect and special effects up to date with G5 onwards, for example it could having [PSN] not KO Pok?mon in the overworld, or upgrading moves to have their PP, power or accuracy as they have them in newer generations.
- This goes without saying and is a standard expectation from me, but having the battles against the important Legendaries be more... larger-than-life-ish. Like, going against the members of the Creation Trio 2 vs 1 and give them the bulk, extra turns or special attacks to merit it, like the bosses in Final Fantasy games.