I agree with buffing WPP for melee, people can completely neglect it now and not suffer much if any consequence.
I think the way to promote team work is the way its done now - you want the multi you have to win the rounds. People naturally do what they thinks best to win the round, the ones who are going to use teamplay to that end will do so. Putting in big bonuses for groups of players will be far too exploitable and probably not result in "teamplay", but in people camping together for the stat bonus or even people wanting to avoid people of similar classes because they don't want a particular "malus" (-6 ath while using a pike?). I like the vague "contribute to the teams victory -> more golds and XP", it gives players a good reward and incentive to win consecutive rounds. I think its a little harsh on losing though, and I would get rid of valour and have the losing team go down to x2 or something.
One thing about the current multi system I don't like though is this anti-team play idea of wearing cheap gear on low multi's. If you want the team to win you shouldn't be encouraged to run around like a peasant, but it seems people are inclined to do that now. I wouldn't mind seeing more gold on low multi's or less frequent repairs, but some sort of other gold sinks instead. Its like people get into a mind set of waiting for their team to get them the multi, and not wanting to spend gold on repairs until then
I wouldn't mind seeing some kind of extra currency besides gold that you get only through having top multi's. Like a currency you only get at x5 that allows you to unlock some cosmetic changes to your guy, or different clothing/dyes. Kind of random suggestion, just trying to think about other bonuses besides gold/xp for winning rounds.
The key to changes to games like this is small changes though and then seeing how that improves the game..sweeping overhauls are rarely a good idea in established games