The Avintia racing team issued the following press release, announcing that Xavier Simeon will be racing for them in 2018 and 2019. Simeon's contract means the 2018 MotoGP line up is complete. You can see the full MotoGP rider list on this page.
the Avintia press release is below:
Xavi Siméon completes 2018 Reale Avintia Racing rider line up
Reale Avintia Racing is pleased to announce a two-year agreement reached with Xavier Siméon that will see the 28 year-old Belgian rider step up to MotoGP with the team and Ducati in 2018.