I actually like the recirculation simulation. Although all kinds of cyclical engines have recirculation of power as part of their function, in fusion there is an important difference from what people are used to. In an internal combustion engine, the crankshaft and flywheel in a car recirculate power from the power stroke to the compression stroke, doing the same thing as the recirculated energy does in this simulation. But in fusion, this 'crankshaft' is very lossy. I suspect if you have a model in your head of how an internal combustion engine works, crankshaft losses are not a big thing. Teaching people that when they model fusion reactors that they need to include this because it's important, I think would help people develop better physical intuition. The 'lossy crankshaft' model was an important part of why I opted for partial direct conversion with the design I built back in the '90s. Set both eff sliders high to see how much this helps.

That said, one big missing thing (other than the economic stuff, mentioned by others) which would add a lot to this simulation would be more about 'where does Q come from?'. Obviously this could be too complicated for a little sim, but perhaps a few simple things could be added like showing how increasing the volume/surface ratio for tokomaks/sphereomaks can help, or how getting rid of certain types of instabilities can improve say mirror or pinch designs. This might help people to understand why certain design decisions (like building ITER so big) were made.