here are some suggestions.
have XP scale based on people in group since dungeon difficulty scales why not xp? this would encourage more pugs even if many players have no problem solo/duo a boost in xp will give incentive to invite players that perhaps can't solo/duo those same quests allowing for a more social environment.
increase the xp jump from normal-hard-elite in epic quests there is a HUGE difference in difficulty from doing say a Von 3 epic normal to doing a Von 3 Epic Elite yet the xp difference is minimal after first time/streak bonus.
And perhaps the xp scale could not be effected by hires so it has to be a real player in group in order to increase the xp doesn't even have to be much say 1% extra of base per each player after 1 so 6 players = 5% bonus for full party. It's really not a game breaker should give it some thought