Isn't it about time that Captain America was promoted? If you go by the Marvel cinematic continuity he's had his present rank for about five subjective years; if you go by comics continuity it's decades. All of which probably makes him the longest serving and most experienced captain in the US army, and horribly overdue for promotion.
And if he's now in SHIELD / some successor organization they should have taken his time in rank in the army into account - again, he's overdue for promotion.
So why isn't he Major America, Senior Agent America, or whatever?