Scrivener would really benefit from a more powerful integrated proofreading and thesaurus module. It has proved handy to provide possible spellings and available synonyms in the context menus of selected words
Regarding German language support the “Duden-Korrektor” is the best available proofreading tool (as it understands the German grammar) and includes a great thesaurus. It may be supplied as a plugin at extra cost (contact epc.de for details). Unfortunately it may be available only for the German language :-/.
Another option may be “languagetool” (autorenprogramm.com) both use these packages to supply a superb language support (Duden Korrektor at extra cost for Patchwork).
Edit: Languagetool and Duden-Korrector both use rules additional to the dictionary, which is much better, then a dictionary-only-approach like Aspell in the Windows-Version. (I just learned, that Hunspell my be another option with a morphological analysis. Hunspell includes a thesaurus, too).
Edit2: Patchwork (which is an Austria based one-man-operation) uses the Hunspell-Dictionaries for standard spelling and thesaurus, LanguageTool for standard grammar and the optional Duden-Korrektor for all of the above.