Usable Leader molds would presumably be
Optimus Prime
Ultra Magnus
Beast Megatron
Galvatron
Blitzwing
Doubledealer
Grimlock
Slag
Most of those redecos have already been tapped out.
Hmm, just as an Exercise, what I'd do if I had to use each:
Optimus Prime - New Head (Previously Micromaster) Overload
Ultra Magnus - New Character, M.A.S.T. (Mobile Army Surgical Transport) named "Lifeline" Medical Deco.
Beast Megatron - Armada Predacon
Galvatron - Straxus remold
Blitzwing - Overcharge is obvious but also kinda dull, IMO. I'd go new (but likewise Quintesson aligned) character again and give him an arctic blue deco call him "Thundersnow" m or something.
Doubledealer - G1 Filch (from RiD15). Do the bird as a raven.
Grimlock - G2 Grimlock easy.
Slag - G2 Slag also easy.
-ZacWilliam, now if SS Sludge was available my answer is "Paddles remold" and every other option can gtfo.