This has been brought up before, often around witness voting behaviors. It would be interesting in my opinion if witness votes decayed over say 6 months, and then had to be manually reset in order to start again - continual opting in.
I'd be really happy with that solution. I think that currency (being current) IS REALLY important in Proof of Brain. Stale is, well, stale.
Thanks again for what's ended up being a really good forum.