Same here. While Having lower level Pokemon give less EXP and high level Pokemon give more EXP is nice in that it allows you to change team members more easily during the main plot, while also subtly discouraging solo Pokemon runs, it made getting Pokemon above level 80 to 100 for friend code battles a pain before BW2's Rare Candy breeder trick. I'd like it if they could either include re-matchable trainers with level 100 Pokemon post game, or just do like they did in DPPt and allow battles to automatically be level 100.
I'd also like it if they could remove those final bits of luck-based tedium from breeding Pokemon; we've finally got guaranteed nature passing, and limited ability manipulation, but IVs still have laughably little controlling them. Again greater manipulation of them, (ideally even without breeding, so legends or male DW Pokemon aren't just playing the lottery either) or making all pokemon effectively have 31 IVs in every stat in Battle tower and multiplayer battles (keeping whatever hidden power it would normally have), would work wonders. At that point, the only reason there would be to make legal hacks would be missing events! (So anti-hackers would need to find a new scapegoat when they continually lose)
It's also be nice if legends could somehow be made into genuine bosses somehow, rather than being slightly more powerful wild Pokemon whose low catch rate means you'll be sitting there spamming Pokeballs all day while your high level Pokemon that resists it's attacks takes basically no damage.