I don't think it would be necessary to top the tweeter personally. A fully veneered enclosure, 3 way design for both towers and center with a wall mountable surround using proven components (drivers and XO parts) would be a very solid speaker. As mentioned perhaps they go bigger with the bass drivers to dual 8's. If hey want an easier to differentiate product they could do a three way stand mount monitor as well.It would seem a bit odd to use the same tweeter in the mid line and upper line of speakers. I think they picked too good of a tweeter to use in the MTS line to be honest....they really can't one up it with any other dome tweeter I know of.
As for the 16" ported, I would love if they came out with something like that. I recently read a post by someone at SVS (sorry for the lack of a reference here) discussing their surrounds and cone size. The post stated that due to the surround on the 13Ultra the cone size is very similar to many 15's on the market, following a similar line of thought a 16.5" driver may well be similar to a typical 18" drivers. Sure the needed box would be very large but SVS hasn't been overly shy of making enclosures the size they need to optimize their products in the past. Add to that the many very high end sub's coming out from the likes of Rel, Revel and Paradigm, there may just be a market for very powerful, very large high end subs that are not merely large black boxes. A ported 16.5" HO, high excursion, veneered, ported sub with a 1000 watt amp with internal DSP, I can see that selling.