I definitely know who Harley is, but who is nate?

Nate is technically Harley in the comics

They are not the same person but they are similar they both help our Ironman from time to time and the both eventually get there own suits. If you count rumors for Harley keener

Talking of Harley, what happened to him after Stark donated the potato gun v2?

You can see him at the end of endgame at Tony’s funeral and in the future there are rumors he will get a suit because in the avengers animated show a kid named Harley has an iron man suit and calls himself iron boy

He appears older in endgame so we know he was not snapped away

He could have been snapped. We don't know his exact age.

Iron lad would open up a lot more story lines,

Why not the sons, daughters of the heroes and kids like Harley and Pete get a heroes of tomorrow style film series

^no. Besides no hero in the MCU has a child (except Wanda in WandaVision).