I still think that WotG would be the best title to pursue from Toho's catalog.
Now... if Legendary really wanted to open up the world they're building, then the sky is the limit. They could bring in ANYBODY from ANY company and incorporate it into their own universe. It really does get you excited to think that we could see Gamera (and his rogue's gallery), Yongary, Guilala, Gorgo and any 'famous' colossal critters from the past interacting in one huge sandbox - without the BS entanglements of companies being stingy or not wanting to play well together (Godzilla and Gamera finally on-screen together, for example). It would be fun & interesting to see them grow a movie universe that could run well into the next decade - or more. Maybe even hold out and renegotiate with Toho for Godzilla once a bunch of other varied monsters have been introduced into Legendary's MonsterVerse (and Toho perhaps runs out of steam with their 'shin' revamp and has let Big G sit dormant again). Then Legendary could really go all-out with a super-duper mega monster star-studded version of DAM!