I believe none except colonising space... The others are so far off that we don't even know for sure if they're even possible, what's going to go wrong or whether we're looking in the complete wrong direction, meanwhile you're dismissing the only realistic one in the next 250 years because we're close enough that we actually know how hard it is.
There's something to be said about chasing after things that are impossible as the possible seems too hard, but I'm not enough of a philosopher for that.