Maybe I'm a minority, but I love a management layer between me any anything. Any sort of software that offers a checklist, reviews, comparisons, contact management, status updates, progress bars, gantt charts, budgets, registries, seating, warnings about conflict or missed deadlines. I should be able to open the "my wedding app" click food, and within a click be able to contact any of my vendors. People helping me plan should be able to read each others communications so they can pick up where I left off. I should be able to add anyone in the wedding party, and even guests, and they see information that is relevant to them. Yelp+Slant+Mint+Intercom for the planning industry sounds great.
The thing that just a budget and schedule lacks is being able to watch each others progress and preferences.
I assumed (and I see there are) all sorts of online tools that purport to help with planning. Many people also set up websites these days. I assume these have affiliate links and the like but they're not exactly disruptive applications.
Some of them all do one or a couple parts of what I described, but they are all designed to be "easy and simple" more than powerful. And I really doubt most of them do things like watch my credit card to keep track of how much has been paid out.
There's plenty of room for innovation, with some more imagination.
The thing that just a budget and schedule lacks is being able to watch each others progress and preferences.