On the outside it sounds like a great idea. Then you realize that some champs who don't need the kill pressure of ignite would take this.
Take Riven for instance. She loves ignite, especially early game. Does she need ignite (especially if her opponent is bad)?: No. She could take the zhonyas effect on top of her other items (may get a GA) . The stasis effect is great on her as far as survivability goes, and since it would then be a spell -- she wouldn't have any negative effects other than a tad bit of kill pressure early since zhonyas was a waste on her otherwise (gives ap, she doesn't scale period with ap).
Take Zed. Another champ who would love that statis. Loves ignite, definitely doesn't need it. God forbid Lee Sin use it. So while your intention was good -- if it's a summoner spell... it's just bad. Overall an alternative idea would be something like lowering the cost of zhonyas a tad bit (maybe a small hit to its stats to balance that effect out).
If you made the cooldown any longer than the cooldown on flash it wouldn't be worth taking .. therefore since zhonya's would be removed for the effect to be a summoner spell -- you'd be in even more trouble against someone like Zed. It would be the ultimate safety defense on champions who don't need the safety of it.