Having libhybris with Arch is mostly a packaging matter, but then you need software which can handle it. For example, only Plasma Desktop/Mobile can be currently used as desktop environment (since they implemented hwcomposer support in KWin-wayland as part of Plasma Mobile project).

I wrote about graphics support regarding Maemo porting before, but even after everyting written in this thread it's still complicated.

Qualcomm has Freedreno open-source driver, which is good, but it's only actively used with DragonBoards and developed against mainline kernel, which you can't run on phone easily due to drivers for other components missing.

MediaTek has Chromebooks, which run on kernel, close to mainline (compared to Android), but the SoCs in them are usually used for tablets, not phones, and they still employ closed-source PowerVR blobs for graphics.

Then we also have less known AllWinner and Rockchip SoCs, which have pretty good mainline support for some of them (to the extent AllWinner tablet was tried as reference device for Maemo porting), but I didn't see any phones based on them, which likely means they don't have modem solutions. Mali blobs are used for graphics (Lima is not in usable status and not actively developed).