Okay, the biggest problem in a game this long is people joining and leaving at any time. We can make some modifications to the formula to make it work, but it can’t be competitive and still be fair. It has to be a cooperative PvE, I think.

With that in mind, here are some things I’d have to do to make this work:
– Create separate inventory world, preferably with visible border.
– Random join locations.
– 3-5 minute mob immunity with a notice for when it starts and when it’s about to end. Maybe just invisibility for start instead? This is needed just in case people join in the middle of the night surrounded by mobs.
– Health regen turned off.
– Regen potions disabled.
– Leaving penalty? Maybe if you rejoin it gives you weakness or something, equivalent to how long you left for? I don’t really want it to affect balance, but it still needs to be fair.
– Scoreboard health
– etc. (Things I haven’t thought of right now)