As anyone who at least casually follows Magic set design knows, creating a block is a time-consuming process. To help mitigate this, much of the work is done ahead of time, in what R&D overlord Mark Rosewater refers to as multi-year plans, usually in the context of seven years.
On Rosewater's blog, he often talks about these plans - and when user northjayhawk asked, "I know you have said that you try not to have +1/+1 counters and -1/-1 counters in the same block, and I do prefer the former more than the latter, but -1/-1 counters once in a while are cool. In the current 7-year plan, will any of the future blocks feature -1/-1 counters?" he answered:
I’ll say this, right now (and things can change) there is at least one block in the seven year plan with -1/-1 counters.
The current seven-year plan is only a little underway, and the fact that Rosewater's answer is subject to change means that the next -1/-1 counter block is more than two years away, since this is the current design turnover time. Still, -1/-1 counters mean one thing in particular is likely - the return of the Phyrexians, the most iconic faction to use -1/-1 counters in recent memory. Whether this means a third Mirrodin block or an invasion of another plane is still unclear. A new setting that has a different flavor justification for using such counters is also possible - much more likely than a return to Shadowmoor (sorry, fans of that plane) since the dual Lorwyn-Shadowmoor setting was poorly received.