The log entry for the final boss in Risk of Rain(quite annoying since I have to like play for about 40 minutes to get to him for just a try at the drop)
Two hard boss rematches in Rogue Legacy
Doing the whole true ending thing in Spelunky(pretty hard, I got discouraged after I messed up very badly shortly before endgame)
Then there's Geometry Dash and FTL: Faster Than Light, which I'm making progress on, even though they're pretty hard, though I'll never 100% them because they have a lot of stuff to.
I think I'll not get involved with beating Ori and the Blind Forest on One Life difficulty, I got pretty adept at it, but not trust myself to last long.
There's also Crypt of the Necrodancer which I'm giving a pass on for now, while the recent DLC is still in early access.
Then I hardly played Brutal Legend because my controller broke, but since I got a new one, I'll got back to it, and am missing just a few achievements.
Oh, and Fire Emblem Fates, beating Revelation on Lunatic/Classic to finish the story, and then beating Birthright for the second time on the highest difficulties to wrap everything up. Thankfully I already did Conquest/Lunatic/Classic , it was really crazy by the endgame...