How about MUTO's taking on the same role as Gyaos?

The MUTOs would probably have to be much smaller and weaker for something like this to happen. A messed up batch of eggs, perhaps?

I can see it now; Goji, Mothra, Rodan, and King Ghidorah are bashing the hell out of each other, then swarms of MUTO overtake them. Sadly, this is probably not oing to happen. :/

My head split open with nostalga when i read this, just imagine it!

Part of me wants the MUTOs to come back just so I can watch the 3 new monsters wreck them, but I'd never want them to be a big threat that requires the monsters to team up. 

^Mirrors my thoughts.

The MUTO, if they return, should be some form of fodder. Not the main enemy anymore.

I'd say that the MUTOs making a comeback in a vein similar to the third film of the Heisei Gamera Trilogy would be okay, but not as a major threat. Rather, I'd prefer small pockets popping up, too much for humanity to handle but now easy fodder for Godzilla after his experience fighting so many monsters in the modern day.