- Type.isType: The idea here would be that we add more functions similar to Array.isArray, like Map.isMap, etc. (It's up for debate whether these are proxy-transparent or not. My view is that these should not be Proxy-transparent, but Mark Miller has expressed the opposite view.) It's important that Type.isType use internal slots, and not an overridable symbol, for reliability (which is the whole point of this proposal).
- Restoring the previous Object.prototype.toString semantics. The original value Object.prototype.toString gives a strong way to check the type of built-ins. This was changed in ES6 to use Symbol.toStringTag, which removes the reliability that it previously had, and adds performance cost, while serving no use cases that I'm aware of. Just like TC39 has recently considered backing out Symbol.species semantics, we could consider backing out the Symbol.toStringTag semantics.
Either of these options may be controversial, but I think we would benefit significantly by making a general decision than several ad-hoc ones on a case-by-case basis to support brand checking.