I am a foreign student living in Germany, who already has basic knowledge of German grammar.
The biggest problem I have now is lack of vocabulary when speaking on a particular topic. If there was a dictionary, or a search system, that could search contexts, so that if I search Schauspieler (actor), it would also give me links to Schauplatz (stage), Bühne (scene), Kostümbildner (costume designer) etc.
It does not have to be the ultimate comprehensive list, but something I could hold a conversation with. There is this pictorial dictionary from DUDEN, which does this to some extent, but it does not have things that difficult to draw, so it mostly focuses on classifications of inanimate objects.
Wiktionary helps with verbs to some extent, where if I search with steigen, I get einsteigen, aussteigen, umsteigen etc. But there is no all-in-one vocabulary builder that I could find. I would not mind paying for it.