Looking at the most recent build of Proton breaking stuff, I think it's time that Valve start maintaining multiple forks to improve the variety of games it works on.

The goal should be to include as many games as possible under two builds (one performance, on rock-steady) then release the other builds to support more things or test new ideas.

For Steam Runtime I would have the two "primary" builds be:
Soldier (default, versitile, good performance/stability)
Heavy (Rock steady, rarely changes, stability enhanced at the cost of performance. Great for reliability and anti-cheat.)

Additionally there should be at least four more consumer builds:
Scout: cutting edge build
Engineer: Extended options available for performance tweaking and getting broken games working without sacrificing performance.
Medic: extended diagnostics and options that cause minor performance concerns.
Sniper: Diagnostic options and flag select fixes that don't cause performance issues.

Finally, two dev builds (which are blacklisted to trigger debug mode in VAC):
Pyro: bleeding edge trunk build with all the newest features. You are crazy to run it in production. Only available to Steam Beta users who accept a warning dialog.
Spy: Excessive debug, includes debug options not suitable for human consumption.

For Proton there should be three primary builds due to the rapid development. I propose using Half Life character references:
Stable:
Freeman: Standard Versitile Open Source Build. New releases are almost always safe.
Combine: Includes custom non-open-source dlls from companies. shipping custom CUDA, APEX, Anti-Cheat, DRM and .Net libraries go here. Not pure open-source
Alex: Quick, agile, new build with recent features.
Barney: Rock Stable and Trustworthy. One build works, all future builds will work Will stay here when proceeding might break anything working. Infrequent updates and long pre-release schedule permit white-list based anti-cheat and DRM to whitelist this build.

Dev builds / Specialty builds:
G-Man, includes one or more new features with "unintended consiquences" that need to be sorted out.
Kliener: Advanced Debug Hook build
Vance: Standard Unstripped Debug build
Vortigaunt: Variant for other builds excluding Combine and Barney that adds a real-time performance profiling HUD. All seeing build used to track down performance problems.