Current multiplayer quests.. even the (theoreticaly) hardes one, The Glow, is possible to do with one player. Our mate did it alone, with 2-3 alts. He collected what was possible and because of that, rest of other mates did not have reason to go there (alone or together). So creating another "team quest" based only at more alts requirement without any other obstacle (focusing on own char or death,..) is waste of developers time.
One thing i see is a problem that eliminates interaction is Faction economy self-sufficiency, almost autarky. Once you are in faction with 5-6 or more players (my experience), a faction dont need to trade with others. Of course, you can i.e. sell surplus blueprints, but thats all.
There is not anything like that one faction has long time monopol to one resource (resource, drug, item, BP, ammo), and another faction has access to different one. Every faction has access to everything (soon or later) and faction who has at one moment access to something valuable (i.e. get first BP of CA, like VSB did), has no need to trade or sell their product.
Faction vs. faction economy interaction is one element that is not implemented in Fonline and maybe it could be the easiest way to boost interaction without reworking of other aspects (player vs. player barter,...).