Where should I wait on you? (If you ask your friend) (Where to wait?)
The modal shall is a must in the sentences like this one below
The Handling agent shall provide licensed personnel for handling these dangerous goods.
You shall be waiting for him at the rear of the building if you wanna catch him up.
Where shall I be waiting for you? means for me that I must wait on you at the place you asked because it is a part of the deal between us.
In addition Indirect speech:
I asked him where I should wait.
I asked him where I had to wait.