>>23473013
My left field wild pick would be Mazinger Zero. Using a series that simply doesn't have Mazinger Z itself present at all feels like a good springboard for a story about Z going shitballs jealous and trying to steal the limelight back. You'd have Z, Zero, and depending on if they get the sequel manga also Great Mazinkaiser, plus potentially Minerva X as a pilot/Support.

For a weird unexpected new upgrade of a licensed mech, I'm gonna fulfill the obvious secret wish of the entire dev team and say Daitarn 3.