Such as the size of the sample library? Is this going to be a huge beast?
Is it a Steinway D or a B?
What kind of controls will the user have? A lot of the newer instruments, such as Ivory, have the ability to scale the velocity to amp ratio, so the user can make the soft strikes louder without changing the velocity scaling. But they don't have other basic controls, such as amp and filter envelopes.
Any control over harmonic content other than through filters, if filters are included?
I know you're hard at work on gettng this out, but a few details would keep us a little less starved for information...