Connor is the best attacker. I can easily think of super levels like his level 1 would be an assassination pounce, level 2 would be a rope dart execution, but not from a tree it would be kind of a rope dart thrown across the stage to grab all three and pull them close to you and automatic kill, sort of like spikes and dante's lvl 2, level 3 would be an Aquila's attack. His minion would be Achilles.
My thoughts were similar
Level 1 = Assassin pounce or running stab
Level 2 = Aquila bombardment
Level 3 = Assassin's guild (all six of them)
Yes to the guy/gal above. Aveline because she's PS exclusive and another female to the roster would be cool.
But to be honest any of the assassin's from the previous games would be great also.