I'd like to see more ingame changes than competitive changes, by making the games more competitively viable.
1. Make every TM accessible infinite number of times. in DPPt stealth rock was only given away once, in HG SS it sold at a store, that's the kind of improvement I want. Still there are tms such as roost that can only be obtained once. They could deal with this problem several ways. Have them sold (money, coins, bp) have some for pick up, some could be won at lottos, through repeatables with trainers or gym leaders, and of course if they continue to use it, the pokewalker.
2. Either allow ev training at 100 or provide something to delevel a poke.
3. allow exact viewing of IVs and EVs in game
I also have a few desires that have a snowballs chance in hell of happening
4. Allow a service to switch a pokemons ability, as well as expand the selections of abilities to be able to be more than 2. I'd like to see Trace Alakazam.
5. Adding in a mechanic to slightly modify IVs. Lets say allowing anyone to add 31 ivs (or 62) to any single poke to fill in lower IVs (but would still cap at maximum of 31 per stat) also there should be an ability to slightly reduce ivs so people could modify their Hidden Power of Choice. This could cost bp.
Out of game aside from new abilities for pokemon I'd like to see the overall effectiveness of stealth rock reduced. I think its ability to hit any pokemon, as well as being able to deal 50% to some pokemon after just a single turn set up is ridiculous. spikes and toxic spikes both require multiple turns set up, both do not effect flying/levitators, and two types are immune to toxic (including one that just removes it when entering the field.) My preference would be to have stealth rock limited in duration, or remove when a rock pokemon enters the field.