Interesting concept, it would be like playing shorter time control over a longer period of time.
Tricky to implement in practice: I play a lot of daily games and sometimes I still think about some of the positions while I am going about my day, so I would still be using more time than my opponent if they limit themselves to time looking at the position.
A nice (optional) feature/setting for correspondence games would be having a secondary time control. So the primary time control is: how many days (e.g 3) you have to play the move and then a secondary time control limit per move (e.g 15min). So in the e.g you have 3 days to play your move but only 15min thinking time.
(While you have the board position open/active in front of you the timer counts down your time.)
This would stop big asymmetries of play time being a factor in games... e.g 1 person thinks for 10min per move -- another player 1 hour.
It would be crucial for correspondence tournaments.
Thanks,
Small Titan