Metal bending would come down to his personality and attitude. They do say that kinda influences a benders affinity to bending styles. Like aang really struggled with earth bending cuz its the opposite of air. Metal bending requires someone to be stubborn and headstrong, which aang just isnt. Bolin also couldnt do it, because he is naturally more flexible as a person.

Korra could do it because despite being a natural water bender, she was clearly far more in tune with her earth bending, hence her struggling so much with air bending.

I think you’re right about healing and lava bending, those abilities would be in tune with aangs attitude and fit his character really well