Might be a decent idea to add a follow up to some hazmat missions with a decon mission. I think it should either be a standalone mission (in the event of some sort of chemical spill) or as a "caused by" mission (similar to officer down, caused by [whatever mission]).
Might also be a good idea to allow for an extension to stations to allow for decon units. I know that in FDNY some engine companies are specifically Decon engines (Engine co. 7 in Manhattan), might be a good idea to have a decon trailer unit that is towed by a utility truck (partially to give utility trucks more uses) or maybe even by the engine (to give the impression of a decon engine).