Look to history, and specifically underrated FX boxes
The Bel BD80 - a fantastic digital delay. Brilliant on Vox btw.
It has a sound and like Kraku above said make sure your box has a sound/character. Stamp your identity on it.
There are loads of verb fx but few really good dedicated delay boxes imo.
As an added feature add in a hold function to the signal being delayed (so it loops). That way it could be modulated, shortened etc in real time. Also add in a trigger feature where the loop can be triggered from either audio or midi. Cool stuff can happen then.
If you want me to completely design it, and there by guarantee you the hit of the decade I will do it for 1 million pounds + vat ....