My preferred candidate is from the House of Orange, either Maria of Orange (if Rupert for some reason is preferred to his sick couisin) or Dowager Countess of Nassau-Dietz. Maria of Orange in OTL was proposed for Charles II, but he preferred a Portuguese candidate.
I also used "morganatic marriage of Rupert being legal in England" for my first TL on this board - https://www.alternatehistory.com/fo...-times-of-first-duchess-of-cumberland.292039/
There was also an idea when Rupert legitimizes his relation with Peg Hughes - the marriage is still invalid in HRE, but in British succession law Ruperta Cumberland is treated as legit Princess. This was used in a challenge "get the (bastard) line of Stuatrts on the throne past 1715" as Ruperta TTL was married to Duke of Lennox, and their descendants later reigned as House Stuart-Lennox.
https://www.alternatehistory.com/forum/threads/rupert-wittelsbach-duke-de-rohan.338299/ This is older PoD, but interesting nevertheless.