Speed ramping should be a quick win, surely? The user interface is already almost there. Just need to add key frames.
My #1 need is some kind of bezier control for ramps, transitions, etc. Add a point here, a point there and bend the curve and voila, complex ramps done. Time this together with keyframes, and you've got some very smooth, powerful controls there.
#2 would be a sprucing up of the interface - some controls (like undo) move around too much, the hit spots are sometimes too small or don't respond to touch as you'd think (like keyframes) and the general flatness makes it feel like an early 2000s app. It's less iOS-y than it could be.
#3 would be an API for a Premiere or Final Cut-compatible plugin system, for more advanced effects. Perhaps the one thing that makes bigger players seem better is that there's an ecosystem that supports them. Allowing third-parties to create add-ons (and perhaps a Luma store to sell them through) could be a great revenue channel for everyone involved. By being compatible, you allow existing plugins to aid LumaFusion (and smaller dev can help make plugins affordable) and then Luma can go on making the app faster and better instead of trying to bundle Cooper Black. Plus, as a user you pay for what you need - standard features are there, and pros can build it out anyway they see fit.