A smartphone is essentially an ARM SBC but with the required radios all in one.
If a phone is boot unlockable and can be flashed.
Why not develope openwrt for phones to function as more capable router than just an Android WiFi hotspot/ tether?
Yes phones vary in specs (instructions and etc). But can't we start with a handful of phones. Repurposing of 4G phones prior to 2015.
And with XDA developers who already have most of the required layouts for the phones.
Are you aware of any phones fully / mostly mainlined?
On Android an Ethernet to USB adapter and switch can be used.
4G LTE WWAN will be the internet input.
Or a phone with USB 3.0 an Ethernet adapter can be the WAN.
The Samsung Galaxy Note 3 for instance is a well mainlined developer phone for a long time now. It's even been repurposed as mainboards for drones. Has a capable SOC that is still more powerful than most router CPUs today, 3GB of RAM, USB 3.0, etc. Codes to radios etc, are available on smartphone developer forums.
WiFi pcie cards even today can't function stably as an AP.
And newly released cards which can, cost from $200 to $500 without the mainboard and with inferior components.
Smartphones can already stably host multiple STA for prolonged sessions like an actual router.
Well, someone -at this point this inevitably means you- will have to start with the development/ porting work. Talk is cheap, but code/ patches/ pull requests aren't - and code fairies only exist in your dreams.
--
Once you get serious about it, the issues and drawbacks become evident and usually disqualify these devices. Things like current mainline support, maintained drivers, I/O performance (multi-port ethernet, concurrent dualband wifi at decent speeds and range (antenna support), 24/7 capable power supply).
I'm just wondering if there are others interested as well.
And just putting it out there to see 
Everything starts with a dream
I'm sure you'll find users who'd consider that to be a good idea at first glance, I very much doubt you'll find anyone capable and willing to provide the necessary code (unless you do it yourself or at least get started).
If you are looking for a smartphone SOC, go with the well supported ipq806x devices, KRAIT300 derived and having all the router specific features a smartphone doesn't have (see above).
To be fair there is more QCM phone SoC support in mainline. There are even some DTS in mainline for phones from Sony and Google.
These phones should have a fair amount of power, but as normal how well mainline actually works day to day changes per device
I'm interested but the effort is to much. Since I am a high level programmer parting drivers is not as easy as creating a web service for me. I would do this if I could.
P.S. if battery, LCD, speakers, microphone or camera isn't working (just one of them) that phone is useless and you will buy a new one. Although that phone is garbage it can be great if you install openwrt on it since LTE and WiFi are fine.
Edit: we can also use some components from Stock os because it's Linux and it has HUGE internal storage (no need for minimal output)
Check with the existing Android ROMs, camera is always an issue at some point, along with wifi, GPS, etc.
You'll face the same issues with closed source drivers they do