If you want truly open source hardware then RISCV is the future.
Unfortunately this is a common misconception. RISCV is not open source in any way that slashdotters recognize. When they say RISC V is "open" all it means is that you can implement a chip design with the RISC V ISA without a license. That's literally the only practical difference between it and ARM.
If you want to use an existing RISC V core in your own custom chip and don't want to make your own core, all cores available now are fully proprietary and must be licensed. Furthermore all the important bits such as the GPU are completely proprietary. So you still need proprietary firmware blobs. Also most cores implement extensions to the ISA which are proprietary.
Cranking out microcontrollers for pennies is neat but it doesn't make it open source.
From an end user point of view, and also Linux support, there's no advantage to RISC V over ARM. They both really suck, frankly.