So it would've deprived Britain of it's most politically-active queen-consorts, but also, Caroline's influence started with her husband's accession to the throne, and ended when she died.
Were she to go to Madrid/Vienna as Karl's wife, she might've given him a son or two (bear in mind, the House of Hannover and the House of Habsburg were in roughly the same straits at the time - George II only had a sister, and two unmarried uncles when he married Caroline; and Karl VI was the last male of his house.)
Oh, and another proposed marriage (albeit unofficially, during the reign of Queen Anne) was for George II to marry Louisa Maria Teresa Stuart. Anne didnt like Sophie of Hannover, since George I had snubbed the idea of marrying her, due to her mother's lowly birth, and she had proposed such a match because she was of the mind, George II wouldn't be able to be king as long as Louisa was alive.
So, butterfly Louisa's untimely death, marry her to George II, and suddenly any Jacobite movement loses some of its momentum. Louisa and her brother, James III, suffered from measles/smallpox at the same time, she died, he didnt. Ergo, flip that if we're leaving her alive, and send her on her way to Hannover (or London) endorsed by the French King and we could get some very interesting butterflies.