Some of the obvious solutions:
-xp bonus for non-merc groups as OP states (sort of exists mathematically anyways as Wayen points out)
-a new race and/or reasons to play lower content
-a new Ldon style expac
But there is easily seen issues with those fixes:
-XP bonus applied to groups benefits all kinds of groups. This includes boxers. I don't have a problem with boxers, but if the OP wants people to make room in groups you don't want to increase the incentive or ease of building box armies necessarily. Besides, join a decent guild and you have groups (is the common reply to this concern).
-Drakkin gave us lots of action in lowbie zones, but not necessarily grouping as the solo-viability of most classes makes self-PLing with potions more efficient than grouping. They could add a new race to Crescent Reach every 6 months and we'd see an ebb and flow of action there. But it appears that selling us illusion items in marketplace > promoting new alts. And they aren't going to disable mercs in TSS, so who really needs a group to get to 75?
Ldon required a lot of population to work. Also, the level spread is so massive now (and shrouding is a failed implementation)...Ldon > than later missions because it was fresh, innovative. Missions bore me to death now and are a huge part of why I haven't progessed in game. A new LDoN might be super-dead if it isn't a loot candyland... in which case it will be super dead once the gear is upgraded.
What has the game lost aside from us really wanting to group, to the point that we meet new people every night we log on?
-if you login the same time every night you probably know the names around you in about 2 weeks... population is that much smaller and loyal. Also people aren't rerolling alts, their mostly getting AA 3024 on their 7th alt. In other words, there is no new people to meet.
That doesn't mean efforts to improve real grouping shouldn't occur, but I think some serious innovation will be needed. What are other games doing that actually works and is fun?
Instances that only pop with at least 4 real characters (no mercs in them) are viable, as long as they don't overdo it or make them the only way to get something. Everquest is best when multiple paths and playstyles can lead to somewhere (i.e., being ready to raid).
Daily tasks that require a group might work... if the tasks are varied and fun, and don't take hours. But the afk in the group movement can't be fed any more carrots as that makes the game feel even deader than it is.
Maybe something as simple as being in zone with a group for 10 minutes creates a +XP/faction aura...
Maybe developing ability/spell combos between classes that have bonuses only for Player characters... example would be enchanter tash line also adding + to piercing damage, 3 melee in a group adds a mana recourse, necro casting splurt/splort line causes % increase in cleric heals...