I think it would be cool if the last mission was a 1-3 mission affair.
And you brought in allllllllllll available troops you could on interstellar transports or have to set up an lz.
Being presented with choices would be cool
A). Attack while we have the element of some surprise
B). Wait for rienforcements, but the aliens will be prepared.
A). Destroy alien supercumputerthingamobbob.
B). Study it in hopes of uplifting your species (possible sequel stuff ;-) )
A). Have cpl hick shave his legs and pretend to be a women.
B). Don't just don't.
A). Add to discussion on this page
B). Don't and begon forever