Fishers fits the profile of two MIC schools (Carmel and Center Grove) very well. I am more on board with your original idea of CG and Carmel bolting for the HCC than I am of this move.
Carmel has 237 MIC championships in the 20 year history of the MIC...nearly 60 percent. They will eventually run out of space in the trophy room and you will see them using their conference championship trophies as doorstops within the building (if they aren't already).
Franklin Central's accelerating enrollment, buy in in the major ball sports, and socioeconomics make them a better fit in the coming years. They should bring Avon (similar in the three aforementioned categories) with them.
Though we all love football, and it is the reason we are on this board, there is more to consider than football. Lawrence Central and Pike are almost non-competitive within the MIC in every sport. They have no business joining the HCC and, as you have mentioned with LC, should be looking for a step down in competition.