I had a look at your proposal but I still do not get why enabling a variable bonding/unbonding time would be a better solution than enabling a variable minimum commission fee (see my comment here)
If I take myself as an example, I picked my validators depending essentially on the commission fee they propose ! I selected 3 validators with 0% commission fee and 3 others with commission fee between 2 and 5%, but this is just for the sake of it. If I felt the need to redelegate now, I would probably pick only 0% commission fee validators. And I do not particularly care about unbonding time.
So according to me, setting a variable minimum commission fee that depends on validator current voting power would be the most efficient way to incite users to delegate or redelegate to smaller validators. Also this rule appears simpler to me and does not appear more restrictive than setting a variable unbonding time. Or did I miss something ? Is there any other pros with your approach ? I admit I certainly did not study this question as much as you, so I am genuilely curious to understand why it would be better.