I don't see why you'd punish a player or make her wait a ton of levels to earn the core ability of Captain America.
I wouldn't -- if I was running Champions. In D&D, there's a genre mismatch. If you want to make it work, great, but the rules aren't really geared towards it.
Let's say you give the paladin the 1d8 damage, ricochet, and returning ability. Let's even say, because it's not really any better than a longbow, you don't require an additional feat or magic item. How is the player going to feel when that one bad guy is in an odd position, without ammo and a chasm between him and the party -- but he has a shield. No feats, no magic item; he just throws it and bounces it twice, critting the wizard. Less severely, do you tell another player "no", when he's in a position where throwing his shield would be helpful? If so, what's the rationale?
I'd go with, at minimum, a feat
or a magic item. Probably the magic item. Feel free to hand it to the PC at the start of the game -- the offset is that he either abandons his concept or he knows he's never getting an upgrade. Maybe the item grants proficiency upon attunement (which the PC would have at the start, but no one else could pick it up and do the cool stuff). I wouldn't feel too bad about him not being able to upgrade because the "bounded accuracy" math assumes no magic items.
As far as the ricochet goes, I wouldn't give him that at the beginning. What makes sense, to me, is that each bounce is an attack, whether or not it's against a creature. So, when he gets to 5th level, he could "attack" a side wall to bounce the shield at the wizard in the back line. If he went Fighter, instead of Paladin, that gives him some impressively complex sneak shots. Wall shots automatically bounce correctly, but bounces from creatures require a successful attack roll (if you want to be nicer, just fail the bounce on a natural 1).
If you wanted to get really fancy with the shield, I'd require the magic item as a base then each new trick is a feat. You might get more than one trick per feat, depending on what it is. Cap's break-fall by landing on the shield isn't potent enough to be worth a feat on its own, but might work paired with proficiency in Dexterity saves while holding the shield.