I don't know if you're planning any of this already but I do have a couple suggestions based on my current frustrations with nextmuni:
1. Favourite buses in addition to favourite stops and prioritise said buses when nearby. I know that 21 and 5 are the two best buses to get me home but when I'm downtown they often aren't even on the nextmuni's nearby list because of the density of buses. I'd love to be able to favourite those buses and have them show up first (or at the very least always show up) if a stop is sufficiently nearby even if I'm at a stop I don't frequent.
2. This one would be a bit more of an involved feature and I'm not sure how feasible it is with whatever dataset and/or APIs you have to work with. Basically, I want to be able to input my destination and have the site serve me buses with stops near both end points (if any exist), giving me time estimates near my current location.
Configuring "nearby" would be handy but far from critical, also.