For those who dont know promote was a summoner spell that was removed in LOL beta,the spell makes a regular minion into a greater minion,even greater than super minions,it was removed coz ppl would spam it and that would result in unstopable pushes.So how they prevent from that happening in Dominion??

Promote stacking is effectively impossible in Dominion due to some modifications we've made to how Promote works. You'll see what those are after Gamescom and PAX.

I assume that you can only promote that big new minion that only spawns once per wave. Did I guess right? :-)

when is promote coming back

It's pretty obvious how it's limited. Notice those tiny melee minions with the club arms? Only those ones can be promoted.