No need to think small when you have eternity to work with. Right now I am building a network operating system, something I've started around 2 years ago. I expect to have an installable and more or less usable system in maybe a few more years.

When I would have an eternity to work with, I'd first look to make our planet redundant. It's insane to imagine that we only have this blue marble to work with, while we have a fireball of hell as a neighbor in our cosmic neighborhood. What even happened there? I'd like to find out. Granted Musk is already figuring out Mars, and he has the money for it while I don't. I don't like the man all that much (too much marketing wank) but hey, at least he's got us covered there.

Maybe one day we could live forever. Maybe we could map and upload our minds. Maybe we could replace our entire body with synthetic components when the frail meat-based components inevitably fail. Perhaps it could even happen in our lifetime, at the pace technology is progressing at. If and when that happens, sign me up!