Any suggestions as to what it should be?
Which factions would be best for a treasure hunt of this sort?
Any ideas of what 'complications' to throw at them?
I'd suggest an item of personal power or something with vaguely evil connotations. Anything that could really tip the balance of power shouldn't fit, or shouldn't actually work.
I'd probably suggest something like an Essence Battery gem that lets you absorb essence from slain foes and store a very large amount.
Memnon should definately be involved, I'm blanking on other obvious factions though.
Complications can include a bill for damages for the Solar the last time they were in Yu Shan, jealous husbands pissed at infidelities, former lovers directly, or someone who they got into a fight with the last time. Lots of fun little ways to make Yu Shan a bitch.