I'd personally love to see a ROM that focuses less on customizations and adding their own "flavor" to Android, and just attempts to track AOSP as closely as possible while supporting as many devices as possible, so users whose devices are no longer supported by their manufactures (or who just don't like the customizations the manufacturers have made to the UX) can continue enjoying the latest Android improvements in UX & security.
LineageOS is great, and already supports a wide range of devices, but all the features/changes it has accumulated over bare Android means it necessarily has a much harder time porting to the latest versions of Android, and supporting new devices compared a ROM without those customizations.
If a ROM like that already exists, I'd love to hear about it.