The Dominion don't belong in a TNG film. Try again.
It would be nice to see Picard do the exact opposite of what Sisko does in "The Pale Moonlight"
But I am completly at a loss to think of any race in ST history* that makes the viewer go, "Are you stupid Picard? Do you know how valuable it would be to have (X) as your allies in the war?"
*That isn't already involved. The Ferengi have too much baggage, the Tholians are too alien. The Gorn are too alien.I dunno.