Are we sure we want this as an Instant? It feels like it'd end games out of no where as is.

Should probably have a template closer to Gilder Bairn, since this gives no indication as to what counters you're doubling.

I mean that's also part of it. You can use this to double +1/+1, -1/-1, level counters, charge counters, loyalty counters on a Gideon, etc.

My point is that if you don't indicate exactly how to double the counters, someone is going to think that their creature with two +1/+1 counters and two Devotion counters can have its number of counters double to four, and decides they can add only +1/+1 counters to do so. There is a reason that Gilder Bairn, Vorel of the Hull Clade, and proliferate are worded as they are.

Or "My creature has 2 +1/+1 counters on it, and 2 charge counters. I need to double that, which will be 8 counters. I think I'll add 4 +1/+1 counters."

I guess we could just have it double only +1/+1 counters. I hate the idea of losing interactions with Charge and -1/-1 counters though. :(

That's not what I'm saying at all. What I'm saying is that this needs to be worded like Gilder Bairn and Vorel of the Hull Clade. "For each counter on each creature you control, put another counter of that type on that creature."