Patreon Character Additions and Fan Made Content
For both Summoned By Accident and GameOver, fan made art and scene content options are available to be added to the games. Details can be found here in this Google Doc.
But, in short, all additions will need to be approved by the relevant creators, such as the artists or writers, and ultimate decisions are subject to approval by Azulookami (and the guest character owners if you want to use them). Other than that:
Scene Art and Illustrations
This one's short and sweet. You have a fair bit of creative liberty on drawing characters and the main requirement is keeping at or below a 1920x1080/16:9 resolution. Cropping to fit the requirements works too.
Scene Writing
You can do player involved scenes, but also third party scenes that don't involve the player character being the one doing the deed. Feel free to use the listed random NPC characters as you'd like, but named side and guest characters must meet their known personality and traits (Shlar isn't going to bottom for you, sorry). And the main characters on the list are mostly off-limits, since they have specific scene goals related to the story. Also, if it even needs to be said, you cannot do any kinks that would violate Patreon rules (and many other rules elsewhere besides). Probably largely stick to the kinks in the games already.
See the Google Doc linked above for the formatting requirements of written scenes. Pages of paragraphed text are not appropriate, you're writing for a visual novel here.
Limited Patreon Character Addition Slots
For both games at a first come first served basis, there are slots available on the GameOver and Summoned by Accident Patreon pages to have your own character choices added to the games. This also includes the option to have specific scenes made for existing characters instead of a character addition or have CG art made for a currently unillustrated written scene that you really love.
Same requirements as the above sections in terms of content, with specifics on what is needed and allowed in the Google Doc above.