Skrmishes are four instances on that same map 1, (2-3), 6 and 12. It is unlikely that Turbine would throw out the existing instance. Try and fit the concept of these instances to limited tech available within the Skirmish system. You are asking for 12 instances which is more than is 50% more instances than are in Helms Deep.
We might be able to get a scalable classic instance for the existing size. Turbine hasn't done any scalable classic instances lately because their instance staff has been busy building the Big Battle system and the eight instances for Helms Deep which everybody has to pay for.
I suspect money - how to charge - for improvements to existing instances is a big issue. Turbine revamped Moria. Didn't do any of the instances probably because the instance staff was working on a money project called Big Battles.
Unless stated otherwise, all content in this post is My Personal Opinion.