An app that allowed flying grid pattern to make better 3d models or linear pattern for inspection.
This is an absolute must, in my opinion.
This used to be a feature back in the copilot days airc. Back then, 3d was just getting fleshed out. If proven effective, it'd be a lot easier than flying similar perpendicular missions twice!
This would be a really handy feature for mapping structures and vertical terrain features. I would also like to see something like Pix4D's Free Flight, which lets you fly manually while it automatically snaps a picture every X meters of vertical or horizontal motion.