I'm glad they're making it stop dashes. That makes it a more effective zoning tool.
However, I don't think this addresses the fundamental problem with the new E: it's a "stun" that should never stun a non-stupid target who isn't already CC'd. Outside of rare situations where a person would prefer to get stunned for about 2s (and possibly eat a full burst rotation) rather than waiting 3s for the walls to collapse, there's no reason for someone to stun themselves on this wall. But that's the only way it can actually stun someone: if they stun themselves by choosing to run (or dash) into it.
A zoning-focused E sounds pretty cool. The problem is that his kit is built around the ability to stun (not merely zone) a target. Especially now that his Q is a skillshot, he can't do a full rotation on someone (or stop them from killing him) unless he stacks his stun on top of someone who's already stunned.
It just bothers me that there's no reason for anyone to be stunned unless they just don't know how his cage works. If you want to give the enemy a decision about whether to run into the wall or not, there has to be some reason why they'd want to do that, or else it just punishes ignorance of how the Champion works with no strategic depth.
Event Horizon needs a new mechanic of some kind to be an interesting, useful skill again. It should not be a "throw this on the ground somewhere and hope somebody runs into it" spell.