Okay, so: I have started two campaigns of my own for the boys. Neither has gotten very far because apparently I don't have the time to put into the prep work, so designing my own campaign is a no-go. (Some really cool ideas, but the implementation... basically didn't get done.) So, instead, I ordered the Complete Roslof Keep Campaign (available here if you're curious) and I'm going to use that.
Meanwhile, Firstborn mentioned the game to one of his friends, and she wants to join the game. Which is actually kind of awesome because we could use a few more players, and now we're going to ask one of Firstborn's other friends if he wants to play as well. (That means Beautiful Wife can skip out on it, which -- given the way her weekends usually go -- is probably a necessity.)
This particular campaign recommends (if not requires) that the characters have their own motivations for sticking with their exploration of the dungeons. It's a dangerous, frustrating place and they're going to run into trouble from other adventuring groups that are trying to explore the same dungeon. So I asked the boys what motivated their characters -- how did they end up in this situation, and what keeps them here -- and after a bit of back-and-forth we came up with the following:
Firstborn (Dragonborn Sorcerer, whom he describes as "trigger-happy"): This character was falsely accused by another member of his clan, and cast out by the Elder. He has come to Roslof Keep to redeem his honor, and so regain his place in the clan and have revenge on his rival cousin.
Secondborn (Drow Rogue or Rogue/Ranger - I'm trying to guide him towards a pure Rogue, because there's not a lot of outdoors in this campaign, at least not the parts I've read so far): Someone broke into his character's home, looting the place and killing at least one of his parents. He has trained until he's ready to seek revenge, but the attacker now leads one of the adventuring companies exploring the Infernal Dungeon beneath Roslof Keep, and Secondborn will need to hone his skills and work his way up until he can face his enemy. ("Hello. My name is Inigo Montoya. You killed my father. Prepare to die.")
I plan to have the same discussion with Firstborn's friend(s), and if it goes well then we're shaping up for a very interesting campaign. I have a few concerns -- Firstborn mentioned that his friend had "character ideas" and then followed that up with the word "centaur", which... isn't ideal for this kind of dungeon-driven campaign -- but I'm betting we can work that out. We're going to need a couple of tanks and hopefully some source of healing, especially for the early levels, but one way or another I'll make this work. I may even add in a couple of NPCs, both to balance things out and to add possibilities for future dramatic twists.
Next up will be the test dungeon, and then the individual negotiations with Lord Aldenmier for the terms of the writs that each character will sign to become a member of the newly-reformed Ivory Scimitars.
Character designs are, for the moment, limited to the core books and Xanathar's Guide to Everything.