In my opinion, a league only needs to be "reset" if it is generally not fun for the majority of players anymore. In terms of your ideas:
- I don't think doing a Keep 10 and Keep 11 for top 2 teams will have any impact. If you really wanted to do a reset to re-energize the league, you could do a Keep 5 or Keep 6 for a year, and then go back to your Keep 12. There will be lots of trading from top to bottom given quality guys you can't keep. And the redraft acts as a way to re-settle things. But this only works, in the long run, if you have managers that have the same level of commitment and skill. [You could combine this with contraction, if there are existing teams that aren't committed, or skilled enough, to warrant being in your league.]
- I do think leagues should have a veto mechanism. The reason being if you have one or two unskilled GMs....it's usually one or two sharks that do all the pillaging. Personally, I see no skill in that. And no fun in it either. If somebody else wants to think they are the greatest GM on earth because they traded three scrubs for McDavid...well, good on them. In my opinion, a no veto system only works in leagues where it's demonstrated that the GMs are committed and knowledgeable (i.e. a competitive league).
Just my two cents, for whatever it's worth.