Notes:

Seriously, it seems there are more bad what-if AUs than good for this show.
Bad End Dippers. Terrible fates Dipper could’ve had in some of the Season 1 episodes.
Gideon Falls Dipper dies at the end of “Gideon Rises”, Gideon takes over the town, and Mabel became his “queen.” Mabel attempts to resurrect Dipper.
Dark Dipper. Dipper teams up with Bill Cipher. Currently becoming more popular after “Not What He Seems”.
"Transcendence AU": http://transcendence-au.tumblr.com/guide. This one has a Earn Your Happy Ending vibe to it (depending on the writer). Also, I would have to spend all day explaining this one since it’s very complex. You should definitely check this one out.
It’s hard to find ones for Steven Universe since most of them don’t have official names (heck, here’s a blog dedicated to Gravity Falls AUs). The only bad what-if AU I could find was the Crystal Infection AU by bonehatter. Most of the ones are just simple concepts (Rose is alive, Lapis is free, Jasper turns good, etc.)