Back on track, I think the big question for a ZX3 is the 'gimmick' or original thing about it. All four Zero games had a special weapon that affected game play by a good bit as well as varying different special abilities and stages that weren't just rehashes of others, ZX allowed you to transform into 4/5/6 different forms (depending on how you look at it) with each one having a very different effect on the gameplay as well as upgrades doing different things, and ZXA doubled the forms you got as well as give each one a unique trait that no others had for the most part.
ZX3 would have to do something different than just introduce rehashed forms from ZXA, but I think a good step forward would be including regular or ride armors and swapping parts out, with each part having a certain amount of effectiveness depending on the other parts (for example, if there was an armor that allowed for flight, one for swimming, one for breaking through objects, and another for pure fighting power, each one would have a set of four parts that wouldn't be as effective if mixed with different armors, and in some cases to help balance things out, conflicts with others, like not being able to fly as far with part X if no other part was used, but if a part for, say, swimming was on, then you'd fly an even shorter distance; some cases parts could have a different effect altogether, like the flying part legs paired with the breaking armors could let you jet forward and rocket punch things). A system a lot like the one used with X in X8, but this time with more parts and uses (and to help keep running around like a mad man to a minimum, have a 'back up' armor set using other parts you don't have to keep returning to the base or whatever (since being able to swap parts out on the fly may ruin parts of the game; imagine going through any MMX game with one armor that nulls spike damage and just walk along the floor and then switch to the other armor to continue on).