To answer your question simply is No. Not many people want a 36" TV anymore unless it's going in the bedroom. Everybody wants bigger and better all the time. Also having a 4K panel in smaller sizes is kind of waste in my opinion. You wouldn't benefit from it as mush a a 55" and up. As for a 21:9 TV catching on or finding one, there is one already but it is not 4k it's 1080p. Unfortunately it hasn't really cached on. I guess it's good for watching black bar blue ray movies, as it would fit the whole screen but for TV cable it doesn't work , it only fits part of the screen unless you zoom it and you wouldn't have a clear good picture.
Vizio XVT 3D CINEMAWIDE® RAZOR LED® SMART TV
Media room- LG 4K OLED EG9600, Pioneer Elite VSX-90, Samsung 4K K8500, Klipsch Gallery G28's, Klipsch Sub 12, Klipsch RS-52 II, Optik HD cable
Living room- Sony 4K 79XBRX900B, Onkyo RC-360,HTPC, Klipsch Reference R-62II, Rc52II,PS4
Gaming room -Panasonic ST60, PS3, Wii