Currently from fusion you're able to einsteinium, uranium, and plutonium in terms of fission materials. There's also a recipe for americium but unfortunately no way to get lutetium for the recipe (so if this suggestion is not ideal perhaps we should have that recipe removed anyway).

It would be nice if there was a more expensive way to get berkelium and californium as well, I think the fusion reactor can be a bit underutilized in this pack (by the time you get to plasmas I feel like solar power is generally what's used in preference for energy), so having more useful recipes to get rare items like this would be great.