Becasue of the ridiculous inbreeding the Hapsburgs did, Mariana (or Maria Anna if you prefer) of Austria was only 31 when her husband, Philip IV of Spain, died. In discussions of what would happen if Carlos II of Spain died at the same time, I've never, to my knowledge, seen this discussion - what if she is the one to remarry?
One might think, for a moment, this isn't needed since they still had a very young Leopold in 1670 but if she didn't think of it before, if he dies in January of 1670, as he almsot did, she would only be 34 and might be able to produce another child. (Although she might be pretty antsy about trying, given the children who ad died so oung beforehand.)
So, would the dowager Queen of Spain attempt to find a man? If so, who would be available? Margaret Theresa of Spain would be even more likely to remarry to try to further the Hapsburgs, and I've seen a few discussions on her if Leopold would pass on, but it seems like Mariana, even OTL, could have at least considered remarriage; or at least if Carlos II died in 1665 as well.