Thank you for taking time to put this on writing! Excited about the future.
Vote expiration for witnesses, proxies, and proposals
This mean if user sets the proxy, they have to renew as well right?! Because if proxy performs governance action, proxied account also getting last action updated means everyone would potentially proxy. Once a year is quite reasonable, especially notifications and email subscriptions could be handled by frontend.
HMT
Would great to explore/research idea of creating token on 1st layer and customizing it on 2nd layer solutions. Like define initial token parameters and assign account to perform future changes only to certain areas, of course supply and other critical parts shouldn't be allowed to change, but with proper migration technique it is possible to create robust system. If token needs critical changes there could be migrating mechanism to define new token id and old token would convert to new token with different parameters, just thinking out load but this could be researched deeper.
Yes, the governance action is separately associated with each account. So a governance action by an account's proxy only sets the last time for the proxy, not for the accounts that proxy to that account. So accounts that proxy must renew their proxy request (or cast a 0-valued governance vote perhaps).
I agree there's a lot to explore on the topic of user-defined tokens. It's one of the reasons I haven't wanted to describe any particular implementation in detail yet: with the many new possibilities inherent in 1st and 2nd layer tokens, we've entered unchartered territory, so careful investigation is required to find the optimal solution.