I don't buy that all there was to Lily/Marshall was building up to wedding/baby stuff. There are plenty of adventures to be had with these two as parents, there's just really no good way to fit that into the show as we know it. They've already basically covered that their lives as parents kind of isolates them from Ted/Robin/Barney's adolescent shenanigans, actually delving into their lives as parents would almost seem like an entirely different show.
But they really do need to get the mother in there. It's not even just a "I want to know who the mother is!" thing anymore. Her addition would shake things up, which is kinda what the show needs. Having a sixth face that you know isn't just a "love interest of the week" would not only remove what feels like an endless wait, but it also adds new blood, a new aspect of the story, etc. I don't buy this BS that the show should end with Ted meeting her. It's not enough. We need to know why this is THE girl- and part of that includes how she interacts with the other five most important people in Ted's life. What makes this girl different enough from Victoria that she doesn't see his friendship with an ex-girlfriend as a threat to their relationship? What makes her different than Zoey, and the countless other chicks he's "fallen in love" with, that Lily doesn't see their relationship as one that's going nowhere? There's stories to tell with the Mother. There has to be...