I'd build an unstoppable bot swarm that could quickly, completely and permanently destroy all social media platforms, news and media platforms, online gaming platforms and AI chat bots. That's the short list. Wherever people are using online digital platforms to become shittier, stupider people, my bot swarm will descend upon it and lay waste for all time. Maybe call it the savior swarm. Ah, sweet dreams.
A mobile OS that I actually like.
A browser that only shows content; like reader mode but better at it.
Physics simulation, to design and then build hardware.