Even if the series ends Season 3 and gets a renewal, the drastically reduced episode count of 13 from the original 22 episode season should open up his availability just a bit for potential Arrow-verse shenanigans.
Without Quantico to hold him back, Tovey would be free to make more appearances in DC shows, and maybe even occupy one of those two hero vacancies in Legends of Tomorrow.
Considering Quantico spent much of last season and the offseason trying to ensure it wouldn’t be canceled, it’s certainly possible Russell Tovey’s schedule may clear up sooner than later.