I think Prokop might do quite well in WRC2 with R5 car. He's not the fastest driver around, but he's experienced and doesn't make a lot of mistakes. Plus he's used to higher tempo of his WRC car, so he might not be afraid to push R5 to its limit.
I don't know much about ERC, but he's already shown he can get into new kinds of races pretty quickly. He finished his very first Dakar in 14th place and now he even did decent times in 12h endurance circuit race in Italy. Jumping into ERC probably wouldn't be much of an issue and I believe he is good enough driver to get on the podium at least in some events.