Author here. Rx is actually built on WGPU[0] which will eventually have OpenGL support too. However, I don't really see much of a future for OpenGL - so building a graphics application today - I would pick something like Vulkan over OpenGL, given the choice.
And to clarify further, the next wgpu-rs version will not have features for backends. Vulkan/Metal/DX12/DX11 will be automatically exposed where appropriate, so there will not be a requirement to have Vulkan.
[0]: https://github.com/gfx-rs/wgpu