Xal herself seems to be set up to be a twist on the Nyarlathotep concept. Which given her vast influences and relatively detatched attitude driven by aliances of convenience is something i would see fitting her well. Her form depending on where she would wish to blend in, with a Ren'dorei likely being the most obvious, as it would allow her to use a portion of her real power, without much scrutiny.
The dagger kinda depends. It's been known to be very slippery, when xal was still within so it still being somewhere out there is not outside the realms of possibility. And let's be honest the Adventurers don't have a great track record with this kind of stuff. *ahem* Kel'thuzad's Phylactery *ahem*. That said the ending of the Ny'alotha patch was rather final overall, in regards to the Old God presence on Azeroth, as we knew it so outside of wink wink nudge nudge kind of hints i sincerely doubt we are going to see much from them, for quite some time.