It's an interesting idea, and I think I like it. I guess it would mean that witnesses would be all be 'canvassing' the large voters when their votes are about to expire. Some might say that it would make the witness list less stable, and therefore more susceptible to a Sybil attack. I don't know enough to have a strong opinion either way though.

Practically, I think there's nearly no chance of it happening though due to vested interests. I expect significant changes to the witness voting system would require a fork of the chain.