The Franks spoke a language median between Frisian and Lower German.
Actually, no. Low German (Saxon) and Frisian are Ingvaeonic languages. Old Frankish was Istvaeonic, and a precursor to Old Dutch. Germans (Irminonic language) love to group everything that didn't undergo the High German Consonant shift together, but that is inaccurate.
Sure, there was a degree of dialect continuum, but the three distinct cultural-lingual Western Germanic groups were well-established by the 1st century AD.
There's two ways of going about this: either split up existing languages by giving them a boost in 'cultural independence' (Scots or Swiss are good candidates, Swiss is so different from Standard German it could be its own language already), or make some more colonial/island languages.
Creole languages are very fascinating, though sadly they tend to die out.
The whole Mennonite or Pennsylvania Dutch angle would be easy to explore as well. One could even have some Argentine German language.