Man, I just love both Infernal Relics and Shattered Timelines, and that's not to take anything away from Rook City either, but I think maybe one of the first two is a slightly better value.
If I could only pick one, I'd say Shattered Timelines. Crono Ranger is one of my favorite heroes, and Omnitron X is another favorite, love the villians there too.
One thing to keep in mind that nobody has mentioned yet is that there is a new expansion coming out: Vengence. It promises to have some really great stuff, let me tell you. I don't know if it will be out in time for GenCon, but you might consider buying it if it is. Or saving your money to buy it when it hits the shelves. You want more heroes (I know you do!) ?, well Vengence has got more heroes! More than the other expansions (less villians though). I'd say Vengence is a strong contender for first-buy (after Enhanced Edition, of course)!
As far as promo cards, I'd just suggest you get the promos for whatever heroes you guys like the best. But if you'd ask for my personal opinion, I'd list them in the following order.
1. Elemental Wrath Zero: Because it actually changes the way the deck is played. Forums are a bit split on this one, people seem to either love him or hate him. I'm in the love him camp, as I ought to be =p.
2. Bunker: Engine of War. I find this is just a great alternate card for bunker for games when his go-to power might leave something to be desired.
3. Dark Visionary: Visionary is the "control" hero and this power makes her Control like no other, it's absolutely fantastic.
3-b. Redeemer Fanatic: I love this card, but I don't know if she's still available. Reedeemer Fanatic is great where Fanatic might not ordinarily shine, much like Bunker: Engine of War.
4. Last Stand Wraith: Love the ability to do damage without any gear, great pickup.
5. Sacrifice Tempest: Tempest is a favorite hero of mine, but I usually just use the original card, but Sacrifice has some cool applications, I admit.
6. Eternal Haka. Also tend to use the original card, but if a card-drawing Haka is what you want, then go for it.
7. Team Leader Tachyon: Has a lot of fans. I list her a bit last because I think she's a bit overpowered =p.
I'll be there at Gen Con in an official capacity as well. I hope we get to play a game with you and your son!
-awp