I like the idea of religion-sanctioned marriages, though there have been a number of high profile inter-faith marriages in BM's history.
I've always been in favor of forcing religions to adopt stances on a lot of topics, even if it's then not backed with mechanics implications. I therefore like the idea of making religions adopt a stance on marriage.
I like that route as well. makes the religion game more relevant. Besides, historically speaking, weren't religious conversions in order to marry a fairly common thing?
So two characters in the same region and of the same religion should have the option to select each other from a marriage drop down menu and send a marriage request which, when accepted by th other person, proc's a new entry into the family history. Extrapoints if we figure out how to work dowries!