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Set up something too big that she couldnt pass it up. Eg new unveiling in the local art gallery or something...

From the proposed heist your different adventure hooks.

Does she do it alone?
Do the other guilds attempt a heist at the same time? who (if anyone) steals the item?
Do the other guilds want to fence it for her?
Do they use force to get her to give it to them?
Do they steal it from her (they are theives after all)
Does she join one guild and is pitted against another rival guild?
Does she get caught during the heist? How will she escape? Does the guild at the heist break her out cause althoguh she got caught they could use her in another, bigger heist! Now she owes them.
Does the owner pay soem master collectors to investigate and retrieve the item before she can sell it on?
Is the item cursed?
Is the item magical? (thats no painting - its "sweet green evening!")
Is the item warded? guarded by soldiers, magic, creature of magic...
Is the item a trap to lure the infamous art thief "Thomas Crown" back from retirement to catch him? Is your PC now mistaken for the old master?

Thats a lot of different avenues based on "shes a thief who tries no steal something". I normally let selfish aligned characters dig (did I say dig, I meant unravel) their own storylines as you can never keep them on track to what youve written.

You know, maybe down the line, orchestrate something like Ocean's 11
Good thief story if ever I've seen one

What you can do is have a high value item that a buyer contacts the thief to steal for them. the item is magical but is not what the thief was told about. So Dangerous the thief needs to start running and keep the item out of their hands (rune Items work great for that).

maybe have them meet with some one in an out of the way inn who can help them out and have the buyer (and several of his minions) trap them in the Inn and slowly pick them off trying to recover the item.