Well so far we have what...
Cap
Iron Man
Thor
Hulk
Black Widow
Hawkeye
War Machine
Falcon
Plus presumably, depending on events of future films,
Quicksilver
Scarlet Witch
Bucky
Daredevil
Luke Cage
Iron Fist
Jessica Jones
Ant-Man
Doctor Strange
Plus whatever they're going to do with Nick Fury, Maria Hill and the cast of Agents of SHIELD. Oh and Deathlok.
It's not a bad number, if they add more to it over the coming years. It's just a matter of sorting out the sides and figure out why some heroes would actually want themselves and their allies to be registered.
Mockingbird comes into play in another few episodes of AoS as well. Not to mention it's highly possible for the Netflix shows to also have an additional hero per show perhaps.
Misty Knight in either IF or LC is logical.
DD could have an array of possibilities for a one off.
All this said I feel it's too soon for Civil War...as we know it.
That's why I think Feige is likely using Civil War the same way he's using Age of Ultron, mostly "in name only" and shaped for the needs of the MCU as it is. Not an adaption of the story we know.
I too would like a Whedon trilogy. The Russos did a bang up job but I'd still like a Whedon Avengers trilogy!