Not even the other couple whose wedding got teased over the summer? The couple who is 2/3rd of OTA - which is the dynamic David was asked about?
Granted I wouldn't want Olicity to get married in the crossover but they're still the most likely candidate. Why would Diggle officiate a Maggie/Alex wedding? It makes little sense that he officiates Barry/Iris' wedding but Maggie/Alex even less so...