I must admit I haven't seen the 90s Mothra trilogy yet, but she always improved a Godzilla movie by appearing in it. Much as I love Mechagodzilla (at least the Heisei and Millennium versions - the Showa one didn't work as well for me) I'm not sure how well it would work in a solo flick. Without Godzilla there, you have to explain why you made your giant robot look like a dinosaur and come up with a completely different purpose for it. Having said that, I'd rather see a MG movie than some of the other big names; King Ghidorah and Rodan never really did much for me. I'd love for Gigan or even Anguirus to finally get a decent story though.
For me, I can see "Kiryuu" as a super robot toku. I would stick with the concept of "Kiryuu". Using the bones of the original Godzilla, but combine some of the Heisei Mechagodzilla with Garuda attachment. His rival could be an update alien Showa MG. I like the concept of the mecha playing the role of "aibo". Heck, bring back Gunhed, Super X series, M.O.G.U.E.R.A (and its mecha pieces, Land Moguera and Star Falcon).
As for other daikaiju...I really want to see a Anguirus, Rodan, and King Seesar team up movie. I mean, they were true blue allies of Godzilla not to mention Godzilla spared them in Final Wars. I can see the three of them playing role of defenders of Japan. Perhaps, the bad guy could be Yog the Space Amoeba starting as Ganime, Gezora, and Kameba, then merging into a new enemy monster ie "True Yog" or something. Maybe Mothra and her children can provide back up to them, but not be too prominent.
