Currently, Twitch can ingest 24, 25, 30, 48, 50, or 60 fps streams. I would like to have 20 and 40 FPS added as well.

Many retro games run at 20 fps, however, the only available multiple for these games is 60 fps, tripling the necessary frame count and wasting bitrate.

40 fps is becoming an increasingly common FPS target in modern console games, as variable refresh and 120hz displays are becoming more widespread. Presently, there is no way to stream at this framerate without stuttering, forcing players to lower their ingame settings to target 30 fps, or upload low quality video.