Revamps - These are done and doesn’t look like they are interested in revisits even though some commanders really need it.
New commanders - Seems the most likely of anything in terms of new content, if at all, and that is presuming they even bother with it.
New maps - Not a damn thing for more than 18 months and it looks like that ain’t gonna change.
New mutations - Should keep seeing them until they grow bored or uninterested and put them back on repeats.
Leaderboards - Remember that? It was going to be a thing for a time but nothing has happened and is almost certainly scrapped.
Other factors to consider:
SC2 will be 10 years old this year and Co-op was out a little over 4 years ago. That’s a long time for any game; how much longer are they willing to provide support before it’s handed off to the Classic Games department?
Player population is very likely down or trending down year-on-year thus it would be less justification for additional new content.
Rate of new content has slowed over the past 2 years or so.
Blizzard communications for Co-op is haphazard at best.
At this point in time, indications are that 2020 is going to be very quiet for Co-op. Sure, things can change along the way but it seems unlikely.
But luckily, 2020 is going to be a bumper year for gaming elsewhere. I know where my attention will be if Blizzard lets this slide.