The proposal would add yet another sandbox-type of (i)frame, similarly to anonymous iframe.
There are already way too many ways how the behavior of an (i)frame can be controlled, so it would be better to think of coherent way to do it all and not add more and more ways.
I'd start by listing somewhere all the ways the behavior can be currently tweaked, and which behavior exactly and how the new proposals work with all the existing stuff.

(I'm not yet sure whether the platform should have the features Fenced Frame would add, partially because the proposal seems to be still very unclear on many things)