I was just looking at the protocol specification. Its still very much a draft but it is based on XMPP.

Threading. Which is what Wave is trying to offer.

Voila! I give you XMPP + threading:

http://xmpp.org/extensions/xep-0201.html (last updated Feb. of last year)

Wave is XMPP, with additional Google extensions for a variety of collaboration features. That is possible because the first letter in XMPP doesn't stand for "XML", it's the (annoying but common) "eXtensible", suggesting that domain-specific additions are not only accepted, they are encouraged.

I'm not sure, it's a random idea based on the idea that most personal communication protocols tend toward being used as notification protocols.

XMPP might be the answer, or maybe dialects on top of XMPP. (haven't looked too close to see what's possible).

please email me, i have a couple ideas i want you and/or somebody to implement regarding notifications ;) aaron.blohowiak@gmail.com