Throw Anything with a Tower Shield

At a guess, if you’re going with Throw Anything applied to a Tower Shield, you’re looking at probably 1d12 or 2d6, i.e. one of the larger, heavier weapons. The weight isn’t even close, but I don’t know that a Huge greataxe (3d6) or whatever is really a good match even though the weight is closest.

Heavy Shield

For Captain America, though, you really want to be able to use shield bashes. You’re much better off using a Heavy Shield (after all, his shield doesn’t look like a Tower Shield or impose the kind of difficulties that Tower Shields do). Heavy Shields can be used as a weapon with shield bash, and can be enhanced with magic as a weapon separate from their magic shield enhancements.

It can also benefit from shield spikes, which are definitely a good idea. Captain America’s shield doesn’t have spikes; I’d ask my DM to allow a “studded” or “reinforced” shield that deals the same damage but as Bludgeoning damage. If you cannot get that, the difference between 1d6 and 1d4 is only 1 damage on average, so that’s not too bad, but the difference gets quite a bit larger as you increase in size (and you want to do that, it’s one of the best ways to improve shield bash damage).

+1 Ranged Shield

But I wouldn’t use Throw Anything; a +1 ranged shield would deal 1d8+Str points of bludgeoning damage when thrown, and return to your hand by your next turn. This doesn’t require a feat at all, which is very good, and it’s a shield enhancement (i.e. cheaper than a weapon enhancement), which is better. However, it cannot be applied to a tower shield. A +1 ranged heavy steel shield costs 4,170 gp.

The problem with ranged is that the 1d8+Str damage is fixed, and it’s very difficult to improve it. Things like shield spikes, weapon enhancements, or becoming Large or larger don’t work. It does count as a ranged weapon, so you can get some bonuses to it, but most shield-specific feats aren’t going to work.

+1 Throwing Returning Shield

At a +3 equivalent, a +1 throwing returning heavy spiked shield deals 1d6+Str+1 damage by default (equivalent, on average, to the +1 ranged shield), but you can add a lot more bonuses to this because size and weapon enhancements matter. The +1 throwing returning heavy spiked shield costs 18,330 gp though: that’s way too much for what you’re getting. It is a lot better than the ranged shield, but you don’t need to spend all that gold on it.

Instead, there are the Gloves of Taarnahm the Vigilant from Player’s Guide to Faerûn, which add throwing and returning to any weapon you have. They’re 10,000 gp, but the shield could be a +1 heavy spiked shield (2,330 gp), so your total (12,330 gp) is still massively less than the +1 throwing returning heavy spiked shield. The savings only improve from there if you add more weapon properties, which you should since if you have none the ranged shield is probably better.

Bloodstorm Blade

A completely different alternative (but a popular one for Captain America, and a good choice in any event) is the Bloodstorm Blade from Tome of Battle. Among other things, it lets you apply throwing and returning to any weapon you like, without having to pay for it. It also has an ability to treat ranged attacks as melee attacks, which can mean very good things for the bonuses that you get to apply to them. Plus, it’s a Tome of Battle class, which means you have martial maneuvers. That makes you more versatile, skilled, and flexible than other martial classes (which are generally underpowered), which is generally a good thing.

I'm sure this won't be the most satisfactory answer, but I would be tempted to approach this problem using my percent dice. Really, the trick is just in picking a suitable "percent chance" that each factor might occur. For example, in response to your points, I came up with something like this:

1. For every 15 feet travelled, roll the percent dice. The ground has a 35% chance (or adapt as you see fit) of giving way to a sinkhole. I'm not a strong statistician, but as I see it, the percentage you select will reflect the percent surface area of your terrain that is covered with sink holes. You can determine the chance depending on how much of the terrain you want to have sinkholes.

2. If the ground collapses, roll 1d4 and multiply by 10 feet to get your depth. Once again, choose the multiplier based on how deep and damaging you want the sinkholes to be. Since characters can ignore the first 10 feet with a successful tumble check, and we want the fall to be damaging enough to be a deterrent (but not deeper than the average length of rope) I created rather cavernous sinkholes. You could reduce the multiplier if you find this too epic.

3. I would adapt the percent chance in (1) to reflect the differing stress the PCs are putting on the terrain due to weight/movement, etc. Make the chance 80% for extremely large or heavy PCs.

4. With a successful survival check (DC=12) before a taking a declared route, let the PCs see your percent dice roll from #1(but don't inform them of the DC/percent threshold for "collapse") so that they can judge whether or not they think the ground will give way.

5. Give the PCs a reflex save (DC=12), but make them check each 5' square in their "retreat" to ensure they are not part of the sinkhole until they reach a "safe" square.

You can of course modulate the percent chances to make it harder or easier to navigate this terrain, but this should enable you to create terrain on the fly.

I don't know if I would modulate anything other than (1) based on encounter level (unless the PCs have taken on more equipment, making them heavier), as the physics of the situation shouldn't change in response to the PC level, but you can make the terrain progressively more sinkhole-ridden, the farther from the path they wander. It is also possible to start adding in creatures of the deep in some of the larger sinkholes to provide an extra challenge.