I enjoyed it as an RPG for awhile. I think Gold and Silver were the best ones thus far. I've not yet played Fire Red and Leaf Green, nor do I intend to. If they want to keep my attention, this is what they should do:

Upgrade to Game Cube or Nintendo's next gen system (you'll see why in a minute).

Battles in real time

More attacks that may be learned and remembered (I like the AP idea, as they progress in level they can learn more and certain pokemon naturally have higher AP than others)

I know Nintendo's really been stupid about this, but support online play. (Actually, I think it'd make a pretty kickass MMORPG if it were done correctly)

Randomly generated trainers so there are an infinite number of trainers you can face, some of which are stronger than you and some of which are weaker, etc. etc.

More TMs and HMs with the ability to buy most of them, especially the good ones.

If you MUST make them prizes at the casino, the ability to buy a lot of coins at once so you don't have to spend hours upon futile hours trying to buy it.

Ability to catch every pokemon in the game, or at very least its unevolved form (encountering a wild Bulbasaur, for example).

Different paths available, such as breeder, trainer, gym leader, etc..

All the worlds can be visited.