This one has been getting me for years and years, and it happens pretty much like clockwork. Group is doing their GNC renbu, and Low and Behold, a Kobold Barbarian! Or some other off-kilter combination of race and class. Now don't get me wrong, having something off the curve is fine, but SO many times, it seems like a good character through session zero, only to hit game, and... it's just the same boring stuff.
Even when I try to engage the point (To use the initial example, a small tribe of kobolds see their barbarous brother full tanking multiple hits from an Ettin, and actually getting up into Melee and taking him to town), there just isn't any interaction there. Like, they're completely flabbergasted to see one of their own who doesn't need numbers, and is fighting literal giants before their eyes, and the character is just like, "Oh... yeah, where's the loot?" and doesn't engage at all with them.
There are plenty of other examples, and varied attempts at the same thing, but I see a lot of players fall into this trap year after year, making characters that should have some interesting stories to them, and the character really wants to explore it according to everything they're saying, but once they're in-game, nothing. They might as well have just done a stock character (Mike the basic human fighter is fine, I've played him before, but I give him that extra personality to bring him out to the forefront.).
What is the best solution to get players to make the most out of these character concepts?