The Houses of Este (ruling Modena, and Ferrara for a time) and Welf (ruling Hannover, Brunswick, and the UK for a time, of course) are essentially the same House, with the split occuring in, IIRC, the 11th century. That is something I always found way cool, especially as Este-Welf has such a distinguished history anyways. IOTL, when Este died out at the end of the 18th century, it was inherited by marriage by Habsburg, with the sidelien Habsburg-Este establishing itself.
So, the challenge is to have Este die out anywhere between the late 16th and early 19th century, and have, despite geographcial distance and different denominations, Hannover inherit its domains, due to that century old connection. That, IMO, would be kinda cool, but is it any possible?