WFM. Be aware that the Cookie spec is currently being revised, and the bar for adding new features is pretty high (there was a previous phase where we solicited proposals, but that's over now). It's not impossible, mind you.
WRT tab-based cookies: One of the biggest challenges is going to be making it usable even when some browsers do implement it, and some that don't. If it's built on the current
Cookie mechanism, that means that existing browsers will treat it as a "normal" cookie and share it between tabs. If it's a new non-
Cookie mechanism, existing browsers won't do anything with it, and sites will likely have to figure out a fallback mechanism.
So, a discussion of how sites will deploy them with this in mind would be helpful. See also the approach to reflecting the attributes of a cookie described by Cookie Prefixes, which might be a helpful technique in this sort of thing.
Finally -- at first blush, I'm a bit wary of defining a protocol mechanism for tab-based cookies; the current behaviour of cookies is not only embedded firmly in the Web stack, but also in users' perceptions of how the Web works, and if behaviours start (seemingly) magically changing, the resulting user confusion could be detrimental to the Web platform itself.
Also, from a privacy perspective, we'd need to evaluate whether this exposes finer-grained information to trackers, etc.