
^What he said basically.
Mothra and Godzilla wouldn't fight. I think they'd have a mutual respect for each other. I think what will happen in 2019 is that Godzilla will fight Rodan, and then Ghidorah's appearance will cause them to team up and fight him alongside Mothra.
"Is that a monkey?"

I'd say it's pretty likely they fight at some point. While Mothra may be "good" she's fought Godzilla to some extent in 5 different movies( I think may be forgetting 1) I think they may fight at some point in the movie. Godzilla is a protector of earth in this universe, but Mothra could be a protector of people and those 2 don't always mix.
If they do fight I would hope for something similar to Tokyo SOS, or the original Mothra vs Godzilla.

...it would look.. stupendous
****
"Must be something we haven't seen yet.."__Bishop
http://www.alien-covenant.com/series/

I feel like the "Godzilla vs. Mothra" title is a popular moniker, ridiculed or not--So audiences may rightly be expecting a clash between the two. I think a quick skirmish isn't out of the question.