shaychana -- S becomes Animagus, horrified & embarrassed to find that his Animagus form is a wolf. Uptight!S, humour, snark, repression, UST, denial, wolfsex. S seduced by R, but no alpha!R.
This is pretty much the plot for my fic The Beast in Me, except for the "no alpha!Remus" bit... I have Severus worrying/fantasizing about Remus as his alpha, but Remus doesn't act like that so much IRL. There is one very large plot point that isn't in the promp, but I'm keeping it secret for now as it's a plot twist for the fic.
stoicstella -- Snape and Lupin must (direly) bring Sirius back through the veil, time frame (during or post war) up to you, excessive plot encouraged, threesome bonus (and rewarded with squees).
Not quite so close to the plot, but still similar to my ideas for The Shadow of Love, follow-up to The Beast in Me. After things didn't work out as planned in The Beast in Me, Albus gives Severus a new assignment: bring Sirius back. Remus is unhappy with the whole situation, Sirius comes back with a very different attitude, and Severus ends up taking pity on Sirius. And yeah, it would definitely end with a threesome!
I keep telling myself I'll get these done before the end of the year, but at the rate I'm going I'll be lucky if I can finish them before JKR gets book 7 out.