I think Gilder Bairn would go well in this deck. It doubles all of the counters that one of your cards has and can keep doing it each turn. 5 counters this turn, 10 next turn, 20 the turn after that, etc.

Same goes for Solidarity of Heroes. While it's only good for one turn, it can hit multiple targets at once if you have the mana to pay for it.

Hardened Scales would help boost your production of counters each turn.

The last card that came to mind was Doubling Season. It always doubles your token and counter production. Be careful not to use Primal Vigor, though, because it also helps your opponent where Doubling Season is just for you.

But, imagine if you will, that one of your cards has a +1/+1 counter on it. You have Doubling Season out and Gilder Bairn. You activate Gilder Bairn to place another +1/+1 counter on your card. But, thanks to Doubling Season, it gets two +1/+1 counters, making it have a total of three.

Next turn, you activate Gilder Bairn again which would put 3 counters on your card. But, thanks to Doubling Season, it gets 6 instead. Now it has 9 counters on it.

On the following turn, your card would have 27 counters on it.

Anyway, just thought I'd mention those cards for you to look at. Hope it gives you some ideas :)

Gyre Sage could be fun with all those counters.

That Gyre Sage would be so fun! Also, I need to update this to include Cryptolith Rite and Doubling Season, it's been a while. Thank you!