Let's add the facts: the 2006 UN catalogue already does not list the (unlimited) silver and black dial Acqua Perpetuals, only the limited black Acqua with different hands and carbon fibre style dial structure.
This is all we have, sorry. One possible scenario would be that one limited edition after the other will be released, so that the market has a steady stream, or better: trickle of Acquas. An advantage of this would be that if you do not like the current Acqua, you just have to wait for the next, which will possible more to your liking. The disadvantage would be for many of us, that until they have saved the money necessary to purchase the Acqua they like, it is sold out again.
The other scenario would be a new Acqua following the discontinued one.
The third one leaves us without any UN perpetual in stainless steel.