Sup. Old member, but new to posting, and just getting into learning to hack and script myself.
I've been thinking about this too, as I'm a perfectionist and love to see my pokemon at level 100, but feel like it's so boring since there's not much to do later in the games, and the last moves for most pokemon are learned from lv50-70.
I don't know about making the grind itself less boring, but perhaps upping the reward for doing it would help. My ideas would be:
-create new ultimate moves for each pokemon that they only learn at lvl90-100.
- maybe add some more crazy hard trainers towards the end, like trainers who have lv90-95 pokemon (which would also make it much easier to level towards the end when you're stuck battling lv50 Pokemon in the wild.
-I like the idea of items that help exp, but maybe to keep the challenge from suffering, make those items cut other stats while worn, like the macho brace, but have a bigger exp gain, but effect other stats. Or cursed items. like ones that greatly increase exp gain, but the wearer is always burned or paralyzed while wearing them.
Also, maybe if you're making your own maps, do something like outskirts to certain towns having unmonitored zones, where common wild Pokemon are at a higher level than normal.
Also, for the ultimate moves, I'd say take it in a different direction than just super-powerful attacks, there are enough of those. some pokemon could have a healing move that can revive a teammate, or auto revive them (but would only have maybe 1PP (PPmax bringing it only up to 2-3). Pokemon known for stat boost moves could learn a move that maxes out a single stat (like Steelix could learn a move that maxes defense. think belly drum without the hp cut) or visa versa, moves that minimize an enemies stat. Pokemon that use status effect attacks could learn a variety of things, a confusing attack that can't be snapped out of. A burn/paralyze/poison that effects the entire enemy team. There could be weather moves that last the entire match. and for fighting types, maybe a move like feint, but if used on a protected pokemon consumes all the remaining pp of that move. Even trap moves that, like the weather move ideas remain in effect for the entire battle.
also, maybe have a few that create certain special items by reaching a certain level.
That's all I got. Of course, as I said, these are more ways to make the payoff for the grind better, thus making the grind less of just an annoyance. this is the only way I can think of that wouldn't make the game too difficult or too easy throughout most the game. If you increase the pokemon stats so there is absolutely no need to grind, then it's easy, if you increase the level of everything else, than it becomes too difficult.
... I wish I were at the level I could implement these ideas...