Hi there!
Honestly, just about any mushroom can be substituted here (Not quite the psychedelic ones, unless that's your kind of thing)
I've stuck with the sjimeji through the years because it gives a nice earthy flavour to the bowl.
If you stick to the base, I've found adding a myriad of things bring some success. Adding tomato paste has been a hit with my friends. Just bang a tsp of smoked paprika in there to give it a nice smokey tomato flavour.