Richarlison, 20, was widely expected to arrive at Stamford Bridge at the start of this season but the Brazil youth international surprisingly opted for a £11,2m (R183m) move to Vicarage Road.
However, the former Fluminense attacker has impressed with the Hornets in his debut Premier League campaign with five goals and as many assists in 27 league appearances.
"In the future, you never know but I leave all things like that up to my agents," he told the Evening Standard. "I want to play in this country for as long as possible.
"It is a great League and has been a great experience for me. I want to improve and playing in England can help me do that."