Hear this idea, a postgame friendship simulator, with some trainers (1 per kind) you get in contact with them ala pokegear/match call, you do some mini-game, missions, battles, etc and grow a friendship with them, the more friendship, the more benefits you will get (items, good trades, etc)
There's a friendship meter (0-100) in the comunication gear if you fail you have to repeat the mission.
He admires you and wants to participate in the pokemon league.
1st meeting: With route1 youngster its a battle at low level.
2nd meeting: You help him to find a specific pokemon (like the farfect'd pursuit in johto).
3rd meeting: You help him to train his pokemon using only pokemon that gives specific EVs, you have to lost the battle but you dont loose money.
10th meeting: You accompany him to the first gym (theres two trainers and the leader) before each one of his battles you give him the right tips to win (from various options of tips) when the battle happens the time fastforwards to the end of the battle when indicates if the tips where right (he wins) or wrong (he looses).
25th meeting: You have a multi battle you and youngster vs other two important NPCs as a comfort for him for loosing to the elite4.
When there's no meeting programed you can battle or trade pokemon/items with those friends.
I'll post more examples