I've noticed that in their YouTube videos there is no theme to adhere to and the structure of the game jam provides the theme itself.

If this is true it means that I can start working on ideas now and whittling them down and thinking about possible logic and code ready for day 1.

That also means that I can grab some free resources and start repurposing them and cleaning them up for myself and those who will use my project base.

I want to leave the most interesting and flexible playground for others on day 1, that interests me the most.

Just wondered if anyone else has some idea as I haven't seen this game jam run for itch before (I understand they do it with other devopers on YouTube)

I might just keep researching the developers to see if any of them has mentioned anything.

This Jam requires a good deal of acting "in the spirit of the Jam" as it is a completely new concept of running a Jam. You are correct with all your assumptions, except maybe the prepwork. The structure of the Jam is the theme. The chaotic joy of taking something someone has started and expanding/finishing/spinning it is what this is about. So if you produce a half finished game on day one, ppl might feel too constrained or maybe intimidated.
As far as I can see, it would be possible to submit multiple games on each day to increase "winning" chances. Not sure if that would be "in the spirit of the Jam" though.

I'll probably give people a bunch of tools and a theme idea, I also make lowish poly models so I'm making a small batch to go along with some textures.

I'll probably craft a solid control scheme with some interactability.

Some kind of example enemy with some basic AI.

I'll write up a good manual and probably recommend going with ProBuilder for the level but its not like I'll force anything and then a few interesting prefabs which give some good functionality.

Finally I was toying with the idea of putting a very blank pause mode with volume and quit options as this can be annoying for some people to put in.

I'm not strong with animations, character creation, sound implimentation or dialogue so I'm hoping there will be a bunch for others to do.