Character ideas: The KDE Konqi, Katie and the whole host of dragons (Yoshi style). Xenia the Fox alternate Linux mascot. Lemmy and Mastodon mascots.

Provisional name idea: Smashtest (like Minetest until they figured out to call themselves Luanti).

Other name ideas: Open Source Battle Royale

Stage ideas:

• Smart Home (Home Assistant themed),
• GRUB (a few traversable lines or options)
• GRUB rescue (one line, Final Destination style)
• Rapid Coding (a DK falls-style scrolling map)
• vim (a close quarters map similar to the Game & Watch map, optionally with walls as the characters are "stuck in vim")
• i3 (a pre-made map styled in a tiling window way, or perhaps dynamically splits the stage during battle)
• compiz (do whatever you want as long as it looks cool, wobbly, wavy, cubey or 3d-like)
• Hannah Montana Linux or Biebian (without a picture of the person in question but the colour scheme and the shape of stage elements would highly imply it's those distributions)
• Blender 3D interface
• FreeCAD animated assembly

Special Move ideas (that can work with most characters):

• Update (a small rest/vulnerable period followed by a buff)
• Mailing List (a beam of emails)
• Harden/Vulnerability Patch (could paint/plaster offensive move, or it could be a curl up defensive move or roll around like yoshi/sonic/jigglypuff etc.
• Release Candidate (powerful offensive move with some drawback or self damage)
• Bug catch (a forceful two handed clap in front, or using a net)

Other attacks:

• cronjob (Tux tosses a clock that explodes)
• Tail whips
• Fire breath

Obstacles, enemies, hazards:

• a bug
• a kernel panic/freeze
• the meditation guru
• firejail
• a green-coloured GPU driver
• git issues
• dependencies/dependency hell
• Blender default cube

Your game doesn't have to have every single element FOSS-themed, as it might make it feel too nerdy, but if you were short on this kind of idea and wanted some, here you go.