I could see the Harpers and Shadowthieves teaming up against the party. Or simply bribing the Cowled Wizards to brand them as deviant and send them to Spellhold as inmates.
And even if they leave the city, the Harpers would still hunt them, so maybe a nice ambush in the Umar Hills or Suldanessalar Forest?
Aran Linvail is going to die in the next two sessions, and the party's about to sail to Spellhold with Bodhi, probably before the CWs can react as they have been ignoring the whole thing.
I rather imagine that when they return, they'll find Mae'Var and perhaps Edwin in charge of the remants of the Shadow Thieves, probably with the Night Knives muscling in on the territory.
Aran has a Ring of Djinni summoning among his treasures in the game; it's ignored in combat, and as a 1x/day summon that is lost forever if it dies, it's risky to use. I'm planning to swap it to "once per week for 10 minutes" with attunement required, as there's quite a bit of spellcasting and damage wrapped up in that summon. Utility will vary based on whether or not the djinni likes them.
Aran's going to summon it with orders to observe, defending itself only, and then after he's dead to report who killed him and why, with full accuracy and truthfulness, to the people whose knowledge of these events will most harm the group.
Right now, that's the Harpers and the Most Noble Order of the Radiant Heart. Going evil does burn a few bridges. The djinni will tell them this later, if they ask.
I have a new player I'm waiting on. One of his options is to be affiliated with the Zhentarim... if he goes that route, they'll be able to call on some less-than-legal support.