The idea of custom mecha is really cool. Some concept characters I've made would totally have their own mecha they started building with their father as a child, rather than a stock one.
The next logical step is the ability to:
a) publish it, and maybe get it rated by other people on the forums
and b) have it possibly appear in future games after you have made it.
I don't know how easy or not this would be to do though. I imagine the basics are already there, I mean the customized mecha you can make must be stored, so it would just be a matter of extracting that.