I would recommend locating unstable wormholes at random locations in the currently known universe (CKU); these worm holes would lead to new, unexplored sectors of space that may or may not be occupied by either human or alien races. The path from the CKU would be unstable, open only on random occasions as yet to be determined; the path back would be stable, so return to the CKU would always be possible, and if the new sector was occupied by unknown humans or aliens they would be able to intrude and conflict with current factions.

Dude, please search the Suggestions forum for these ideas, a variant of your idea has been suggested before.
-1.

I did see a suggestion with a variant, but I believe this to be different enough to suggest it thusly.

It's "different enough" for you to need to think harder before barfing this crap into the forum.