Well right now it appears force field will not be back seeing as how there are multi-tiered arenas. And also it appears that stage fatalities will can be done at any time. But I believe the MK: Team has designed these level's to were to will take come effort to get to these "Death Traps".
(As you can see in the Slaughterhouse Death Trap there is a railing around the edge. So I think this indicates that once you step on the designated square you will have to pull off a certain juggle move to propel them high enough to even achieve the Death Trap finishing move.)
But I think for the most part in a part of an arena were you're supposed to be closed in, there will be walls of some sort.
(Now I believe this is below the sky temple. Judging by the scenery in this courtyard Im willing to say that this is an area completely closed of, as if this were the bottom tier. But however the railings towards the gate look somewhat destructible, so anything can be possible.)