Mop Up Any Remaining Quests, Especially Companion Ones
While The Outer Worlds doesn’t have any endgame, per se, there’s a lot of quests to find and complete, and it’s very possible you haven’t done everything by the time you beat the main story.
Because of that you should go and mop things up, and if there are any major side quests you haven’t done do those, then beat the game again. Some of the side quests can affect the ending as they feature major decisions.
Past that, if there are any companion quests you haven’t done, we cannot recommend enough that you finish them. The companion quests feature some of the best writing in the entire game, and do a great job of fleshing out your crew members.