The Phyrexians of Scars of Mirrodin block were conceived of as a way to bring poison counters back, which they accomplished with their two primary mechanics. The first and most infamous was infect, which probably is loved ardently by as many people as it's hated virulently. The other is proliferate, which actually seems to get love across the board; after all, it's much more open-ended than infect and outside of a poison context can do fun things like making creatures with +1/+1 counters already on them stronger, advance quest counters on enchantments and level up already-leveled creatures, and, of course, increase the loyalty of planeswalkers.
In a non-Phyrexian context, therefore, the return of proliferate is something many players might look forward to. When penguinofhonor asked Mark Rosewater on his Tumblr blog, "Would you bring Proliferate back in a block without poison (or another type of counters to give to players)? Or is giving counters to players too important to Proliferate's design?" Mark replied in the affirmative:
I could easily see proliferate coming back without poison. It would have to be in a set where it matters but there are many ways to do that.
Since proliferate specifically references counters on players, there would indeed have to be another mechanic alongside it that added counters to players independently - and Mark's comment has opened the doors for new design space that may or may not include the infamous "gold counters" (which seem to have been precluded by Gild).