TL;DR;
"Hold Action" is not part of the game
Highly complex to add, if even possible
An option to "Delay Action" would suffice
Options: Act After "_____" / Reduce initiative to "X" / Go Last
So I'm pretty sure adding something at potentially complicated as the ability to Hold your Action would be prohibitively complex but would it be possible to create the ability to Delay your action until later in the turn? Perhaps lowering your position in the initiative order until after another pc/npc, reducing your initiative number or even something as simple as going last would be a much better than simply having the choice of "act now" or "lose your turn" which is what we have now without the concept of "Hold Action".
Honestly, no clue if this is even possible but wanted to get thoughts/ideas from people who might have enough knowledge to know.
Thanks.