-wen is a moderately common Germanic feminine name element with a sense of "desire, belief, hope".
Ros- or
Rosa- is a Frisian and High
German form of Saxon/Nordic hros/hross (by metathesis Old English and Frisian hors), rare as a name element, but is recorded in names such as Roselm, Roscetel, Roschil and Rosalind/Roslyn. Rosewen is conceivable then as a Germanic name.