Honestly just allow people to delay their turn, permanently moving lower in the initiative order at the cost of their reaction. It's vastly similar and fulfills a similar function.
- Doesn't require any action/bonus action/movement-lost considerations that Readying does require
- Doesn't require any automation/decision tree to cover possible readied actions: attack first enemy seen? attack a specific enemy? under what conditions?
- Already basically implemented in DOSII
Failing that, Ready should temporarily move you in initiative order (hopefully to a place of your choosing) and remove all your bonus actions and movement and reaction, leaving only your action. Having specified triggers would almost certainly be either way too limiting (e.g., Solasta), way too complicated (full decision tree for the conditions you'd use your readied action), or too intrusive (pop-up promts: "Do you want to use your readied action now?")
