I'm running OpenWRT on a mini PC with 8 GB memory and 256 GB disk.
I know that the current build of OpenWRT is designed for running on embedded systems with limited memory and storage. However, I'm evaluating the idea of building an image for more beefy systems. For example, including more comprehensive packages, such as full-featured vim, more diagnosis tools, etc. In fact, I'm thinking about building it like a typical Linux distribution to avoid things like storing config files in /tmp. And installation/upgrading will be done like Ubuntu/Fedora.
Is there any obvious reasons that this may not be a good idea?
It's not going to happen, as it would create too much of a split between 'normal' embedded targets and x86_64. The only way to 'survive' (to keep it maintainable), is to keep OpenWrt behaving the same between targets - either you lose support for all the devices with constrained system resources and make it x86/ SBC ARMv8 only or you keep it as is, targeting traditional routers and just happen to support x86_64 as well (and very well, at that).
If you want to go more towards the later, Debian, Gentoo, arch and the other more modular/ server oriented general purpose distributions are already mostly there. But at the end of the day, it's still just a router -not a client- or workstation system-, so less is more. It reduces the attack surface and concentrates on the things that matter for routing. Yes, that means anything more than a ~256-512 MB USB stick/ sdhc as main storage or more than ~1-2 GB RAM is kind of wasted, but OpenWrt will still shine with those specs for what it intends to do. It's a router, not a general purpose server or desktop.
Openwrt is great for low power x86 machines like thin clients, it mostly outperform any soho router. But if You want to do more then why not to try pfsense / opnsense / vyata/ sophos ? I'm personaly use pfsense as router and it works great on two cheap thin client configured in high avaiblity.
General purpose distros can provide a qualitatively higher level of integration, security, extensibility, scaling, and support by utilizing full featured system/init managers like systemd, full featured package managers like DNF/APT, mandatory access controls with SELinux/AppArmor, system wide cryptographic policies, full scale virtualization with libvirt, etc.
While OpenWrt focuses on networking, routing, firewalling and supporting as many platforms and devices as possible, it cannot compete with general purpose distros on their field, as this would require to abandon support for everything but high-end devices, replace the core tools such as busybox, procd, ubus, netifd, etc., and basically create a new distribution based on full featured and full scale solutions mentioned above.