There is no benefit for any current ECC school to make a move to the GMC.
If you want some rivalries.... make week one Kings vs Mason, WW vs Princeton, and maybe a Loveland vs sycamore (just due to how close the districts are)... Any other games would just be games. Now I agree, half the ECC could compete in the GMC at the level of a Colerain, Hamilton, Middletown, Sycamore but there is no reason why a Lebanon, Milford, LM etc would want to leave where they are.
Colerain's only rivalry in football is LaSalle that is just down the road. As long as they play them close that will be one they keep playing for a few more years
If Harrison keeps growing and with the disbandment of the SWOC they could compete in most boys and girls GMC sports right now outside of football.
Edgewood fields Varsity, JV and some freshman team sports. They would be competitive in ALL sports in the ECC, Ladies sports in the GMC and compete with the bottom 5 schools in all sports in the GMC.. However they are moving to the southwest buckeye with Ross where they can be very competitive.
Talawanda has some nice facilities and outside of football could compete with most of the GMC or ECC schools in all sports. They like Harrison are on the outside looking for a league. Talawanda also fields Varsity and JV in all sports even a Ice Hockey Team like Mason, Sycamore and some GCL schools have.