"Open ended" in the sense that since the epilogue scene takes place thirty years later, if anyone wanted to go back and do more Disco that takes place immediately after the events of S5's storyline they could and have thirty years to insert any kind of adventures in. And indeed, based on the finale's epilogue, the only things they need to be mindful of is that thirty years later Michael, Book, Tilly and Admiral Vance are still alive and Disco itself is still around. Otherwise, they have as blank a slate as they would had they been doing a sixth season or whatever.
Granted, aside from the Academy series we're probably not going to see too much in this time period in the near future. Another season of Disco isn't happening, a Disco telemovie likely won't happen given Section 31's tepid response has likely put a pause on any future telemovies. Even the novels might be under instruction not to do any Disco stories in this time period so as not to interfere with any plans the Academy series might have. Not that there's very many Trek novels these days anyway.
