Some of the current thinking for Sentinels of Echo City regarding Armor Class and Invulnerability:
Armor Class for supers is a base of 10 + DEX modifier + Level Modifier. Most supers don't wear armor or have invulnerability; they just have that base AC. AC has to improve a little as characters level, or they will eventually always get hit. The Level Modifier bonus keeps things relatively low-key. Spidey at level 8 (about where I'd put him) has AC 10 base + 7 (from his DEX bonus) +4 (level modifier) for a total AC 21. He's almost impossible for a common thug with a pistol to hit; that thug needs a natural 20, and won't get to crit against him. That makes sense to me.
Armor Class can be improved by the body armor talent. This gives a base +1d6 to your AC (in addition to the modifiers above - roll this at character creation). A character in chain mail armor might have a +4, while a character in heavy plate armor takes +6. This is the best way for a character with low DEX to improve his AC.
Invulnerability doesn't impact Armor Class; instead, the character ignores the first so many points of damage per attack equal to Invulnerability. This would also be rated +1d6 for a starting character; at Invulnerability 6, you effectively are immune to small arms fire (since most small arms weapons deal 1d6 damage), excepting a critical hit. The Hulk probably has invulnerability 10.
Some characters will have both Body Armor and Invulnerability. Superman probably just has Invulnerability (albeit a high degree of it - and his high DEX bonus makes sure he isn't hit too often), while Iron Man has both Body Armor and Invulnerability (albeit medium amounts of both).
A character could break the system: level 7 (so +4 level modifier) on top of AC 10 (base) with high DEX (maybe a +6 bonus) and body armor (let's say +6 for fun) has AC 26. We could balance this by saying you get the better of EITHER your DEX bonus OR body armor (since heavy armor would prohibit movement); this aligns with the existing rules, makes sense, and enforces some game balance. In this example, the character has AC 20. Personally, I'd trade out the body armor for a shield in this case, since the shield gives +1d4 to AC while you carry it, so the character could still have up to an AC of 24. This is a character who is heavily invested in defending himself (and might be along the lines of Captain America, although Cap doesn't have quite that much DEX; he's probably at +3 to his modifier, but 1 level modifier higher).
We could complicate things SLIGHTLY by allowing a combination of DEX and body armor, up to a maximum bonus of +10. This means that Cap gets +3 from DEX and +3 from wearing that sleek chainmail, +5 from his level modifier, for a total AC of 21 without his shield, and AC 26 with it (he's upgraded his shield over time). He is VERY hard to hit with that shield in hand. It also means that if you put Spidey in a suit of heavy plate armor, he's only getting a marginal bonus from that armor, since it's slowing him down and actually neutralizing part of his DEX bonus by its nature. I actually like this option better than the paragraph above. Make it so.
By the way, this all came out of play testing my character Tribune (think Mr. Incredible with flight- I posted a pic of him yesterday), and having him lose half of his hit points to a few very lucky rolls by a group of thugs. Sorry, but six thugs are not going to almost take out Mr. Incredible. Not happening. Just a few points of invulnerability (let's say 3 points) makes it 50% likely that small arms fire bounces off of his skin altogether.