The hardest parts will be obtaining print cartridges, rollers, and fusers. Designing a standard to run off a certain vendor’s hardware will be a pile of issues, and nobody will just start manufacturing hardware for a handful of hobbyist printers.

The 3rd party generic cartridge market has this solved. You would design the open-source printer around a popular cartridge platform that will probably exist for years to come.

Cartridge designs are longer lived than you expect: Brother and HP sell many small variations of their toner and drums (or toner+drum for HP) under different model numbers that are made incompatible with only small bits of plastic to block their usage in the wrong printer. Generic manufacturers label their cartridges as working across several models as they’re actually intercompatible if you avoid those bits of plastic (or add extra mechanisms to suit both shapes), with varying levels of success. Source: I used to maintain printers across a few sites at my previous job.

I suspect the real problems might lie in the militancy of printer companies. It’s a dying market that they’re fighting over each for, so they’re used to awful tactics. I expect they wouldn’t be afraid to sue & block open printer projects using patent laws; there would be nothing worse to them than printer hire companies who supply big business suddenly providing in-house printer models.