No idea where this fits on the priority list, but I think a lot of problems around stream processing still aren't solved, and it's holding us back from a really productive programming paradigm. Handling updates/retractions elegantly is hard or impossible on many platforms, handling late (sometimes _extremely_ late) data can be very inefficient. Working with complex dependencies between events (beyond just time-based…
Martin Kleppman's articles and research is a great place to start for anyone interested: https://martin.kleppmann.com/2015/02/11/database-inside-out-...