I originally had the Green Dragon and the Manticore over on the Sanguinarium side to give Van and Jottun more choices. I moved the Green Dragon over to the Supplicants side as I felt it more in keeping with the serpents/poison theme of the death team. But DrP, if having the Green Dragon available for Vanheim/Jotunheim will satisfy your team, that's no problem. The death team can certainly live without it.
Alternatively, as rdonj and DrP hinted, I might certainly consider allowing certain thematic rainbow pretenders for each team as well (the ones that do not conflict with the other main paths of teams) but yes you'd be restricted to a single path for simplicity sake. So a great Warlock with only blood magic would be fine by me. As would a Great Sage for the Usurpers restricted to Astral. As would a Crone with only death on the death team, etc. If we wanted to complicate it a bit further, sure you could also add some secondary path magic as well, no problem, but no other paths would be allowed on those rainbows. I did want to preserve the merc's water capability, as the Usurpers secondary path is also the main path of the merc. But if Zegc, doesn't object I see no problem. The merc is not a team strictly speaking and is not playing to win. So I have no problem with single or primary paths only on these rainbows.
So your choice guys. Green dragon or the addition of selected rainbow pretenders with magic path restrictions to all teams. If I go the second route, I'd remove the Carrion Dragon, Dracolich, and Vamp queen and any others that conflict. The Gorgon is out, not only because of the earth magic, but it is one hell of a good expander. Don't want to give DrP's team too much.
The pretender choices should be able to be verified fairly easily, so I am not really worried about that. As sombre noted, RPG first, balance is second. But I want people to feel comfortable.
Just to note:
There were 3 considerations I was looking for with regard to pretender choices:
1 - I wanted each team/faction to have different pretenders (no two teams should have the same pretender).
2 - I wanted these pretenders to fit in with the theme of the team.
3 - I wanted to preserve the main paths of each team as much as possible at the start hence the exclusion of the cheap (10 magic path cost) rainbows.