Something I was thinking about while making my first GMO:

Graphic Model Overrides that also affect Crosses and BeastOuts. These would then effectively become Graphic Model Style Change Overrides, or GMSCOs (a better name would probably be best, but for now it'll do).

Now obviously not all GMSCOs would be compatible, but this would make for some interesting combinations. Also, these GMSCOs would have to be registered when the relevant combinations are created, just like normal.
GMOs can be affected by crosses. For example, the MachFighter.GMO changes whenever Machman activates a cross while the MachFighter.GMO is active.