Many ROM hack players have a certain feature that's non-negotiable for them - if a hack doesn't have something, it will make them much less likely to try it. As examples, some people won't play a hack that doesn't contain the Physical/Special/Status split, or Mega Evolution. I don't have any "non negotiable" features myself, but I do love to see a custom region! It's awesome to see what kinds of worlds people build and explore a totally new and unique set of towns, cities and landmarks.
It's not really a feature as such, but I prefer a game that feels fast. So typically that means short dialogue, few cut scenes, either quick or little backtracking, and no more than a few mandatory trainers on each route.
If we're talking about real features. Hmm… I've never seen a hack with it, but automatically fully-healing your party after each battle would guarantee I'd try it.
Auto-run. Once you've experienced it, there's no going back. Holding the B-button or fast forward just to move quicker is a pain.
I think an expanded bag space is also must. Being only able to only hold 42 items and having to use the PC to store the rest is just annoying. There's isn't much worse than finding an item in the middle of a route and not having space for it.
I think the physical/special split is a must-have feature nowadays. It's part of the core series since 2006, so that's 14 years now. It's really awkward to go back to 3rd gen mechanics.
Otherwise, I personally prefer to play games in new regions, rather than playing a re-imagination of Hoenn or Kanto. But what's more important is the polish. It should *feel* like an official game. Gaia does this really well, among others.
I personally prefer hacks and fan games that will at least show me IVs and EVs with exact numbers and no graphs like in base game Sun and Moon.
I like it even more if there is a way to improve IVs in game. I played Reborn with a mod that made it so when I maxed an EV stat it could reset the EV to 0 and improved its IV stat by 1. I thought it was a really good design choice for the mod so that later in the game I could grind my pokemon I had always been using to be strong enough for the end game fights like the gauntlet.
Propably working codes. Things like codebreakers, action replays and gamesharks. I'm using "cheats" to bypass designed limitations.
- Wild modifiers. Don't want to play around with starters that are (once again) forced on me. I would rather gather a small team on my own. Don't need help. Advanced Starter won't change scenario in my case.
- Don't want to "catch'em all". And especially legendary ones for that is simply dumb. I remember playing Gold remake not to long ago. Developers there were pretty much forcing pokemons on the player - even legendaries. For what reason? If you remove a core of a legend from a legend, then you're also making this pokemon world soulless. Legends should be -by my own standards- unobtainable. Out there for imagination only - and also to keep things in perspective. But never reduced to a battle item or record in a pokedex.
- Walk through walls/No dark-like effect. Don't want to use stupid HMs like "flash", "rock smash" or "cut". These are a waste of space! And why even Pokemon are doing everything? This stupid trainer is just using them left and right becase His Path Must be Unobstructed? I believe that this trainer of ours should do something more than just use others. If you can't progress on you own then what type of a champion are you? Paper-made tiger? Hence i usually gather HMs and then use walk-through walls on rocks, bushes and water. I have the designed option unlocked, but won't touch it. My dude can bypass silly obstacles and swim. He can't fly; he ain't from Baam so no wings on him. Flash doesn't make any sense to me and never did. Every damn phone in our world has this basic feature called "flashlight". Don't tell me that this hyper-advanced pokedex can't even do that...
- Nature/EV/IV settings. I can save an abstract amounts of time during ev/iv training with codes. This is not for competitive purposes outside the game; just for my pure entertainment. It's not a very important thing but a feature. A small thing to do really, so why wasting hours after hours?
- Teaching techniques that are by default unlearnable to a certain Pokemon. I like "Bullet Punch". I would like if Aggron could use that. He can't. Unless i use codes. I mean, the move makes sense so why not?
Those are all just a small details but for me those are opening many doors.