Githzerai are basically built on one solitary character for the longest time, though I think there's a prominent githzerai in Baldur's Gate 3. Their coolest hook isn't being made to be monks, but using their willpower to mess with Limbo ... except this is pointless in the Prime Material Plane (or natural world, etc) and it's not usually part of their character sheet anyway. I also see them as a reflection of the githyanki, little more, or one of many victims of mind flayers (there's so many of them!). If there's no githyanki, there's little reason for githzerai. (By contrast, you could write an entire novel about drow where surface elves never appear. You could do an entire novel about drow, entirely in the Underdark, where no non-drow appear.)
To me a firbolg is something for a GM to play with. I'm completely unfamiliar with the 5e firbolg, however, just recalling them from 3e and possibly 4e.
Wildhunt shifters are cool for the Eberron setting, but IMO they're one of the lesser lights. They're too tied to the Church of the Silver Flame. There's also not really a place for them outside Eberron. Mechanically I think they're pretty cool, especially in 4e.
Kalashtar are also an integral part of the Eberron setting. If you're not using psionics, there's really no place for them. Furthermore, a big part of their lore is just opposing another faction. I don't think I ever looked them up in 4e. (One of my 4e campaigns featured psionics, but that was a Dark Sun campaign. We didn't have kalashtar, warforged, or anything like that.)