I mean, it's always ok. Whether it's a good idea is a different question.
Take for instance Mass Effect. It was a series that was beloved, but the ending to the third and Andromeda left a bad taste in people's mouths. So EA needed to kind of respark the series. A big part of that was the Legendary Edition rerelease. But... the fanbase may not be too excited to replay Shepard's adventure knowing it was going to have a crap ending and no future.
So, before that was released, they showed a trailer for a new Mass Effect game that implied that the story would in some sense be continued. They let people know it wasn't a dead story/franchise. And, I wound up buying the LE, which I think I may not have done if I didn't have some hope for Mass Effect 4 eventually coming out and being decent.
With that being said, I kind of assumed that when they showed the trailer, it would be out by now. The initial excitement I felt had kind of subsided. If they said it would be out within a year of the trailer, I'd probably have preordered right away. At this point, the excitement is died down, so I'm probably going to wait for some reviews and maybe even an eventual price drop before playing it.
So, there's a balance there. I think it's best to be up front and clear. Preferably give a projected release year, or at least make it clear from the beginning that something is not coming out for a long time.
