First, if you owned KI, would you also "own" Cedar Fair? If not, you lose a lot of the synergistic budgeting that is potentially available by running multiple parks. Going on the premise that it is KI by itself, then Banshee may be it for awhile when it comes to large rides. The waterpark really could use some nice modern slides/water coasters. That would likely be my next move after Banshee.
I do not think KI needs a token Giga anytime soon. A Maverick-type coaster themed to aliens/space would be after Banshee. I'd dump Firehawk and put the new coaster in X-Base. A family-friendly Wild Mouse with spinning flying Saucers would be cool too. Have all of it glow at night.