I think it would be great if could set up your own shop on the road, or big cities, or in the mountains to feel like a guru.
You should be able to sell pokemon and items, and set a price for them. Other player come to you and buy, no need to trade.
This should work while you are offline, as well as online, but imagining the server stress, maybe everytime you setup shop, you have to relog to play again.
This might also give some possibility for pvp. Like if there were specific spots for selling, and not everywhere, you could claim
that spot from another player. The shop master should have a strong team on him. I don't know how the AI works though, so maybe that's impossible.
Any way, i know there are some holes in the things i suggest, but never hurts to suggest things, right? Maybe something can spark in the imagination of the developers or the players.