According to NFL Networks Ian Rapoport, the current belief is that both Demaryius Thomas and Emmanuel Sanders will be back with the Broncos in 2018. He states their run for a top quarterback(Kirk Cousins) as the reason for both of them being back.
This contradicts other recent reports indicating that the Broncos could lose not one, but both receivers this offseason. The Broncos have a contract decision to make on Demaryius Thomas and Emmanuel Sanders is reportedly on the trade block, so the questions about both remain.
I would assume that the Broncos and Thomas will be able to work out something that frees up some cap space for them and keep him with the Broncos.