This is maybe weirdest idea, but there is it.

This race cannot directly interract (fight) with anyone else, and anyone else cannot fight it directly. But, this creature can affect areas, temporary affecting it. Principe is the same as changing weather, but much more effectively: turn room into a holy place, which would harm evil creatures; or into a blessed place, which would cure other characters - and etc.

When the room changed, obelisks, crystals, stones appears there. When they are being destroyed, ambiants are getting hurt. Both, mobs and players, can destroy those objects.

Ambiants can be still harmed by some area spells magic, though they can't fight back physically.

Many of their affects aren't working on mobs - only in PvP.

So, they are allies, healers, saviours - but not solo playing class. They are getting observe and explore xp X10 (only in XP regards, not in edge points bonuses) - this way they can learn alone.

Very much like ghosts, they are immune to many dangers, but probably still can be harmed by some mobs, or cannot enter explore/dangerous areas without companions (players) to balance it out.

Yes, I don't think anyone will ever bother to think about such utility class due to difficulties, but I think it would be most interesting change in CF (such classes) and could bring a lot of new strategies and teamplay.