You know what you should make as Luna's gundam. Something like the Strike Freedom Gundam with the Meteor weapon system, or the Strike Noir Gundam, or the Double X Gundam. I'm sure one of those three Gundam would be perfect for Luna. And Cadence's Gundam could be like the Infinity Justice Gundam with the Meteor weapon system, or the Nobel Gundam, or the Rising Gundam.
Actually, I think that the perfect Gundam for Luna is the RX-91GP04G Gundam GP04G Gerbera. After all, Luna IS Celestia's younger sister. Since Celestia has the RX-91GP03S Gundam GP03S Dendrobium Stamen, it would make perfect sense for Luna to have the next Gundam in the GP series.
And this also means that Nightmare Moon's MS would be the one that the GP04 was remodeled into: the AGX-04 Gerbera Tetra. (Since Luna and Nightmare Moon are the same pony, their Mobile Suits NEED to be related in same way.)
And as for Cadence...judging by the fact that she (and Shining Armor) practically used the Sekiha Love-Love Tenkyoken(you know, the move that was used to finally destroy the Devil Gundam in the last episode of G Gundam) to defeat Chrysalis...I'm thinking that the God Gundam is the best pick for her.