You mentioned X series. I don't know why, but remembered X3.
It have built-in scaling system, but not all players noticed it's existence, as it smooth. It does not lean on player level or any other constant-progressive value. All the missions, including plot, rely on combat and trading ranks. If player choose missions far above his current rank, or doing actions (like destroying enemies or making significant profits by trading) his rank is growing FAST, giving more challenges. Once player starts to accept missions below average related to current rank or unable to do progressive-difficulty actions, his ranks will grow slowly or even deteriorate. So, if something happens, like you loose your main flagship and unable to make bounties you used to do before, - just wait and do something else - soon rank will degrade till you can match.
That system is great, as player always get exact difficulty it wants to play. If you want easy-relaxing game - just do not choose missions harder, than you want, stay where you are. If you need a challenge - go on, beat unbeatable and next challenge will be harder! Game will generate something insanely-hard for hardcore psychos, if you are.
That is suggestion.