Data Driven Pages is awesome. There are so many efficiencies gained from using this feature, but sometimes not every page in the mapbook has every feature listed in the legend.
My idea is that when Data Driven Pages is enabled, there should be an option for each legend item (in the legend properties) to only list features in the legend that are actually visible in the extent of that page. A checkbox to activate this live legend feature would be ideal because perhaps there are base features you want in the legend regardless of whether they're seen or not (like maybe roads, or streams).
Maybe you have a 25 page DDP mapbook showing wildlife obervations, but Sasquatch habitat are only in 9 of those pages, Jackalope are in 23, and el Chupacabra are in 14. You may only want the features seen in that specific map page in the legend so the map user isn't looking for a feature that isn't there.
I think this would be super useful, (even outside of DDP). It should be simple, all the tools for this to occur should already be in place... Lets make mapbooking even more efficient!