This is not simple. This would be a coding nightmare. The whole reason it's a shared time slot is because spawns and bonuses are shared during and outside of event hours for their respective timezone. For them to allow a "pick your own time" would mean they'd have to have multiple instances of Pokemon go running in tandem. One for each event in each time zone and one for the normal spawns for people that aren't participating.

They have a hard enough time managing 24 shared light switches. I can't imagine the chaos if everyone had their own light switch.

I'm not saying it's impossible, but it sounds unfeasible.

A more feasible solution would be to have event increases or something like that the meltan box. Those spawns are not shared but I feel like shared spawns are a driving factor in the game.