So here's the concept. As the campaign progresses, without telling them (or at least not directly telling them), the party will hit "checkpoints" within space and time, reappearing in that exact moment should they TPK (which I will ensure happens at least a handful of times), resetting their level and equipment to as it was in that point of time, but retaining the knowledge from their past life. Aside from letting them use this ability to be killed by a seemingly unkillable boss, only to respawn and pelt it with a specific damage type they found it is weak to, how can I make this interesting?
Two ways of giving a greater narrative to this that I have thought of would be to either have variations each time (slightly different terrain, loot, monsters, etc.), or by making the world slowly become more and more "corrupted". Maybe starting out with subtle changes, like variations in the flavour of foods or the colour of flowers, and gradually building up to more drastic changes, like a mountain becoming a grassy plain, and eventually the 3 dimensional space of the material plane collapsing in on itself in certain places.
Through all this, I have two uncertainties. Firstly, what is causing all this? I can see this just kinda being the way the campaign works without explanation, and I don't think that'd be dissatisfying if I wrote around it, but writing the origin of this into the story could be really interesting, and then cumulate with the party deliberating on whether or not they should try to end the time loop. Secondly, I am unsure what the actual story would look like. I wanna run a sorta zombie apocalypse-style campaign at some point, but I'm unsure whether the time loop gimmick is a gimmick that can just be slapped on top of a zombie campaign and then pressed together to form a story, or whether I should try writing a story specifically about the time loop (which leans into my first uncertainty), likely involving aberrations and other eldritch horrors slipping through the cracks in reality caused by the corruption of the loop.
Any and all feedback would be appreciated. Thanks :3
(Also I am posting this on my alt cause one of my future players knows my main reddit and I don't want them getting spoilers lol)