One thing I'd like to see, but I'm probably not going to be able to do (though DS game hacking is getting better, it's definitely not something I'm good at) would be a Platinum Sinnoh Dex Edition. Basically what this would entail would be:
-Bring up the missing wild Sinnoh Pokemon in Platinum, and fix Spiritomb for emulation (since one can Pal Park with any good DS emulator on PC or mobile rather easily and similar projects exist on the GBA games, National Dex isn't a huge problem for Platinum outside of it's own region. Concerning flashcarts, I think they can still Pal Park? Not sure about that.)
-Trade evolution fixing (easily done honestly, you can rig this in Universal Randomizer without randomizing anything.)
-Event items picked up/given and flags set at the time of obtaining them (The hard part, for me anyway. This would probably be all legendaries including Azure Flute and those Platinum-only Regis/Rotom formes.)
-Selling of items required for evolution, unobtainable berries, and TMs not obtainable again (Battle Frontier-centric)
I doubt any big sprite replacements or bugfixes would need to be required, as IDK if any fixes were found yet anyway. I guess a nice bonus thing to see would be uncensoring of swimmers or full decap (if that's even possible at this point; Gen IV handled capitalization a lot better than previously done anyway).