I just want to say up front I have no knowledge of future development plans, this is just based on what's happened in the past. Since the game has launched, there has been one big update each year, starting with 1.1 in 2016. So if the pattern holds we'll see a 1.6 update in 2021. However, the pandemic may have slowed things down. We got 1.5 only right at the end of the year, and depending how big a potential next update is, it could be early 2022 by the time it's ready to go. Or it could be a smaller update so it would be ready to go earlier in the year.
As for a "2.0", that really depends what you mean by that. Sometimes when people say that, they mean a really big update for Stardew, but since that would probably mean a much longer wait time before the update was ready. Given that the development of updates has been pretty steady since launch, I don't image we'd see significantly different types of game updates. And ConcernedApe has said he never plans to charge for DLC or anything like that, so I think that rules out something like that. If you mean 2.0 as in "Stardew Valley 2", we know that isn't happening, at least not anytime soon, but that's not to say there aren't new things coming.
As for what else is next, ConcernedApe is currently working on two new games. One is set in the same world as Stardew, but will not be farming game or a sequel. The other will be related in some way, but it sounds like the connection may be even smaller.
He recently said he may reveal what he's working on later this year: