Hesse is not a cohesive cultural region, and has never been, at least not in the form shown above. Even in the Merowingian times, the post-Roman South was culturally entirely different (and more similar to other regions along the Rhine than to the Hessian North) from the Old Hessian regions in the North and Middle, and those differences morphed and morphed but persisted throughout the Middle Ages. After the Reformation, confessional divides cut three-way riight through the land.
Culturally, there is little cohesion: neither linguistically (various isoglosses divide the region into varieties clearly differing from North to South), nor economically (the urbanised regions along Rhine and Main vs. the resource-rich woodlands in the East and North vs. the agricultural breadbaskets / pighouses of the Wetterau and Schwalm), nor even culinarically (in the North, regional specialties are a friable sort of salami, another sausage that's slightly reminiscent of haggis, and beer as a drink; in the South, regional specialties are sour milk cheese and a sour cream-based green sauce, and wine and apple cidre are favourite drinks).
Of course, in pre-modern times, any sort of territorial mix-up is possible regardless of cultural cohesion, economic relations or whatever, so why not - but then you have to ask yourself if you want to achieve it AGAINST the Prince-Bishopric of Mainz, which has possessions all over the place, or IN THE FORM of this Prince-Bishopric, i.e. having it gobble up the entire land (but then you'd have it united with lands outside of Hesse, too).
