D&D 5E No Thief/Acrobat in XGTE, so... how would you do it?

[Sacrosanct]

Make them a Rogue subclass option...as they originally were.

Gets you Rogue's, well, everything but specifically, Expertise (make Acrobatics & Athletics), Uncanny Dodge, & Evasion

Subclass features
@ 3rd: Grab Thief's Second Story Work
Grab Monk's Unarmored Defense.
@ 9th: Grab Monk's Slow Fall.
@ 13th: Grab Fighter's Remarkable Athlete
@ 17th: Grab Monk's Unarmored Movement

Done.

This seems like a good solution.

 

[akr71]

Keep in mind that the 5e thief doesn't have a whole list of class only skills (pick pockets, climb walls, open locks, find traps, etc), so there is no reason to create a new subclass like the acrobat that does do that.

As others have said, expertise in acrobatics and athletics is about all you need. Even though they don't get Expertise, monks are a great candidate for an acrobatic sneak/fighter.