What other monsters do you think could show up in this year's movie?

Well the Oxygen Destroyer is confirmed so I'm guessing if we don't see Destoroyah outright we will see a glimpse of his smaller forms in an end credits scene

Other than that I think it will mostly be OC kaiju

Just the confirmed monsters seen in the trailers, and some more original titans

MegaGodzilla?

Maybe some surprise original monsters (like some surviving M.U.T.O.s). But who knows? (Insert my forum signature here.)

Godzilla's_Tail79: Well isn't there a long-running rumor that Destroyah is going to appear in Kong vs Godzilla. K and G will fight for a while then team up against Destroyah? No idea whether there is a grain of truth in these rumors, but I won't have an issue seeing Destroyah on screen again, although of course it has to be portrayed well on screen.

Godzillatheking123: That would be a massive rip-off of BvS but if it's done right (or better at least) then I'm in. Sounds like a great finale to the series.