Fenris would sooner run for office in the magisterium than join Solas plan to let demons, spirits and more magic into the world.
Velanna, dunno with her or the elven wardens, at the end of Inquisition the warden command structure seems pretty damaged, if not completly destroyed. With the wardens either getting exiled from the south of Thedas followed by a warden civil war in Weisshaupt or the southern wardens splitting from the northern wardens.... followed by civil war in Weisshaupt. It seems that any elven warden could join up with Solas if they wanted to, although like it is said in DAO you can't run from the blight (not counting Fiona) in the end, either you find it or it finds you.
Zevran is too antivan for team Solas.
Merril, hard to say. In my mind it might depend on whether Hawke was a friend or rival to Merril. Although I am leaning towards her not joining Solas at all.
It would be funny to see Anders reaction to Solas plans...... hehe. I hope they bring him back for the next game just for that.