Just my own 2 pence or cents.
I'm fairly new to gamedev and asset creations, but I have uploaded some of my own assets in packs themed after my own game jam games.
With regards to the content, I will only use the assets I have created and therefore have full rights to use, so no headaches for copyright or licensing.
So far I have only uploaded them for free as the corresponding games are also free to play.
With regards to the structure of the packs, I always try to keep the files organised in relevant subdirectories for simplicity.
With regards to the file types, I try to be consistent. For 2d images and sprites I use PNG, for audio, I try to use mp3, for 3D, I would include exports for the common file types and the BLEND files I used to create them.
If there is a lot of files to be included in the pack, it might be useful to include a contents list or readme that can help a user find particular assets more easily.
With regards to updates, this all depends on your own purpose for the pack. If it is a standalone pack, I would feel it is complete when everything you want to make for it is in the pack. If an update to this includes separate file formats, this could be done as a separate collection under the same store page. If the update is more unique assets, I would either look at a DLC like model or, if there is a lot of new assets, a completely new Asset Pack. One of the things I have realised is that there needs to be a defined cut off for when something is considered to be finished, otherwise you will never "finish" it.