First off, Retropie is fantastic, and I really appreciate the hard work of everyone who contributes to the project!
For anyone who wants to play Retropie in a vertical orientation, it’s a well documented problem that rotating the whole system in /boot/config.txt causes performance issues.
However, Retroarch and MAME4all-pi both allow rotation in their configs, so if there were a way to rotate the EmulationStation GUI (system/game select), we would be all set! The command-line wouldn’t be oriented correctly, but I think this would be a fair trade-off for improved performance when playing games vertically.
I’m not sure how easy this would be to do, or if it’s even possible, but I thought I’d put it out there! :)