I know I'm not the first and probably won't be the last to suggest this, but I think one thing that would really help expand the dynamic of the game's immersion would be to include some races from outside the PHB (Gith being the exception) such as goblins (or hobgoblins), orcs, lizardfolk, tabaxi, kalashtar, and so on. As someone who delights in using as many different races for as many different classes as possible, I know I would get a royal kick out of playing my goblin barbarian as she goes throwing her fellow goblins around like enormous flails, or my kalashtar cleric of Kelemvor who smites undead without a second thought one minute then starts mumbling to her quori spirit in front of her party the next.
Of course I understand for narrative purposes that there are limits, like I know I can't hope to play my warforged paladin who is basically Optimus Prime in a fantasy setting because that li'l tadpole ain't gonna find much flesh to gnaw on in there (unless you somehow find a way to create warforged mindflayers. Now that's a haunting thought).
Still, while I know there are plenty more races and classes to come before the game is fully complete, I would love to see this kind of ingenuity some time in the future, pre or post-completion. It's a great deal to ask for, I know, but one I hope you'll either consider or have considered already!
Love you guys! Keep up the terrific job!