There's lots of open source RISC-V cores - but can't be any fully OS ARM ones (even if someone's made a n OS ARM core you still have to pay ARM a royalty)
The big advantage to rolling your own private ISA is that there's already a rich compiler and O/S ecosystem for RISC-V while if you do use RISC-V it's all already there for you.
It really does feel like a second generation 'barn raising'