itch.io

Are you suggesting that it could be a free download or that individual USB drives with the OS pre-installed would be sold?

Why not both? I've seen several Linux distros do this. It's a good way for them to make some money to fund development, but it doesn't stop you from downloading it and putting it on your own media.

Second, what architectures should be supported? Obviously x86 would need to be supported as the vast majority of computers use this architecture what about other architectures.

Everything desktop/laptop is 64bit these days.
32bit mean mostly old hardware and few "recent" netbook. The bad things about that is that OEM have been very lousy into support Linux's hardware driver in the past. That's very true for gpu hardware (with only nvidia doing the right job, but restricted to it's private proprietary garden), meanwhile the coming for Vulkan seems to be a "game changer" for ARM and 64bit hardware with 32bit architeture left for a very marginal role.
So, in short:
32bit - Good to work at maxium compatibility (probably it could run even on the "junks" that run in your local libraries). But missing the potential of last/more expensive PC/gamingPC (ie: no more than 4GB ram)
64bit - oriented to latest pc hardware that will run Vulkan compatible GPUs (also support for greater RAM quantity). Optionally promote it as "benchmark tool"

if itchOS had ARM support for example then would make it a highly versatile system as it would be able to run on many other things such as the Raspberry Pi (or similar boards.) A mobile version of the OS would also be possible but this is probably overthinking things a bit.

It's a bit difficult to see Raspberry Pi and similer as gaming platform other than retro style games. As mobile I would like to see how things go with the latest SoC that are getting out from AMD/Intel/Nvidia. The failed kickstarter campaing of SmashZ was supposed to come out with 8GiB of ram (pro model); I guess this mean the AMD new SoC is supposed to be 64bit also.