Biggest: largest, most ambitious, most lucrative, riskiest, highest impact? I have one that meets all of those criteria. Ditch legacy and redefine one language/OS to rule them all. Give app stores the middle finger. Throw the web browser away. Throw away antiquated build systems. Create a universal, minimal, extensible, and sensible layout and rendering engine. Create an imperative language as fast as c yet portable at the code level. Oh, and expect no one to invest in your madness. :) Be Steve Jobs and throw away the floppy disk and CD ROM before anyone else.
Look at the millions of complaints filed about HTML, JavaScript, CSS, etc. There is huge room for improvement in the way we write native and web applications - the fact that we still make a distinction signals its own problem. We can do better, and there will always be room for more innovation and improvement.
