For example, there is strong devices like with 2.0GHz 4 cores. If the device had a HDMI-like output interface, we could possibly turn into a PC right? With video and mouse and keyboard? I mean it would be lean system.
No. OpenWrt is not designed as a general purpose OS. There is no proper graphics support for OpenWrt, no drivers for graphics cards in their actual graphics modes, and none of the libraries and the like to enable applications to actually draw to the screen. The best you get is text mode/CLI.
And sometimes people ask a followup question... "what would it take to add it?" The answer is simple -- it's not going to happen. It would be an enormous effort and would end up making OpenWrt extremely large/bloated in the process.
If you have a spare PC that you want to use for a general purpose computer, install any of the many flavors of desktop class linux.
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I've seen Doom on menuconfig so I got curious. Also some xorg packages too like wth haha 
Whyy xorg
Yeah, the question comes up from time to time. I'm not sure, actually, why those things are in menuconfig, they'll never actually be usable on OpenWrt.
Is there a system similar to openwrt but with a gui?
I would say Alpine. Very light.
The OO can run Doom, and I’m sure others but, . . .
That's a good choice, as it's the native source for apk (Alpine Package Keeper).
I set up a laptop with Alpine about a year and a half ago just so I could play with apk, but ended up using it for my travel computer, too. Only drawback is that there are some DRM packages that are glibc-only binary blobs (Alpine is musl, like OpenWrt), so I couldn't watch certain video streams while on a trip. Oh, well...
there’s nothing really stopping you from installing a desktop OS and then running openwrt virtually
you can also try running openwrt in a container (docker / systemd-container) but this is not supported / recommended by the openwrt team