Baldur's Gate would probably be the better choice then Elturel since there is so much recent information published about it. Elturel, from what I remember in 4E, was pretty much an uber-paladin stronghold. There were some hints of big brother style oppression, but for the most part it was pretty undefined.
Yeah, I've read the 4e FRCG entry. I'm inclined to ignore the oppressive aspects, especially since HotDQ makes no mention of it, and it doesn't seem like it would be a place where a fun-loving paladin who likes drinking (eg. Ontharr Frume) would be particularly welcome.
What I'm thinking I might do, though, is run the 4th mission from DDEX1-1 in Elturel. That's the one with the blue dragon tooth that has a weird lightning spell effect thing that leaps from person to person. I was planning on introducing an Emerald Enclave rep in Elturel to go along with Leosin and Ontharr, and that mission seems like a decent way to do it. I can either use the half-orc in the adventure or replace him with Delaan Winterhound (the EE rep from
RoT).
Then I'm thinking I might run the first mission (the one with the exchange for a fake dragon egg) in the Outer City of Baldur's Gate. Maybe even in Blackgate. That one involves a Harper agent, who I could say was directed to the PCs by a message from Leosin. The third mission - the one in the polymorphed dragon's crypt - can be set in the cemetery in Tumbledown. I can keep the priest of Kelemvor and everything, and he can still introduce himself as a member of the Lords' Alliance.
I'm liking the idea of introducing all the possible factions early on, instead of just the Harpers and the Order of the Gauntlet. I can see the dwarf wizard PC wanting to join the Harpers, and possibly also the elf arcane trickster, while the goliath druid will find the Emerald Enclave most attractive. The human noble fighter I could see going either with the Order of the Gauntlet or the Lords' Alliance. Not too sure. Can't see anyone wanting to join the Zhentarim, which is fine with me.
Anyway, I'm also pondering the possibility of running the fifth mission from DDEX1-1 as well. Maybe instead of rogue Black Fists, the guys running the secret prison could be rogue Flaming Fists, and if the PCs end up reporting the prison's existence to the Fists, Ulder Ravengard will taken an interest in them, which will let me introduce him earlier than the council meetings in RoT. And he may invite PCs to join the Lords' Alliance as a result.
Having a fallen noble in the party could be a pretty good hook to get them back into the Tyranny of Dragons story line. Maybe her family has been indited in aiding and abetting the dragon cult? She gets back home to see the place surrounded by the Flaming Fist and has to prove her family's innocence, or maybe find out for herself how far the rabbit hole goes?
That's a possibility too. I'm not sure how personal I want to make it. It's possible I'll just let her use her status as a patriar to open otherwise closed doors (as per the noble background feature) and such. It'll certainly allow the PCs to wander around the Upper City at night, for one thing.
I'm wondering if it would be worth trying to work in some kind of meeting between the PCs and the dukes, where the PCs have to try to convince them not to interfere with the cult's movements so the PCs can continue to track the treasure train. But that might be better left to more influential faction agents. Perhaps that can just be something that Leosin and Ontharr mention in passing -
"We'll get some of our established agents in Baldur's Gate to smooth things over with the powers-that-be so they don't accidentally tip off the cultists and disrupt the flow of treasure heading north."
Aside from Leosin, who spent some time as a prisoner of the cultists, the PCs are the only ones likely to recognize any of the cultists from the camp near Greenest, so it'll be their job to keep an eye on the people passing through Blackgate. But they could also potentially hang out in some other places, like
The Blushing Mermaid, in hopes of catching a glimpse of a familiar face there.