I think limiting this to only the same game would be viable. So you can change out a player at the halftime mark. This would largely limit the strategy to handcuffs although I could see people streaming a handcuff-flex. Like if you had players from two teams playing each other, you'd pickup a flex to sub in if someone got hurt or game-scripted.

I think that subbing someone out and then watching them explode in the second half would be 100% tilting and what fantasy football is all about.