The Traveller: New Era (T:NE) setting offers many advantages for sandbox-style play, among them limited interstellar authority, an actual "hard" explorable frontier, and a good framework for adventure. However, personally, I care little for the T:NE "house system" rules, which tend to be more complex and less suitable to my tastes than Classic or Mongoose Traveller. So the question arises: If you want to play in the rich, dynamic Traveller: New Era setting, what ruleset would fit it? Somewhat ironically, I think that the Classic Traveller (CT) ruleset (as presented in the three Little Black Books and in the Traveller Book) fits this milieu perfectly. Let's have a look at some of the features of CT which work great with the T:NE setting or similar post-apocalyptic space-opera settings:
First, Book 3 world generation creates a very wide variety of worlds, usually very different from each other in technology and population, and sometimes having very strange combinations of starport, population and environment. Sometimes very low-tech worlds would sit right next to highly-developed ones. This makes less sense in a well-developed, stable Imperium than in the post-apocalyptic Wilds of T:NE. Why do so many worlds have TL 3-8? Because of the Collapse. Why are there higher-tech worlds around? These are recovering worlds. Why a Starport-A and TL-F but only 1,000 inhabitants? These are the last surviving people on a previously heavily-populated world, wandering the empty halls of their old starport and obsessively maintaining it in hope that, some day, the Imperium will return. Why so many nasty dictatorships around? With the central Imperial government gone, each world went its own way, and many had to enact harsh measures to survive. So an "out of the box" subsector from Book 3 could be a perfect T:NE setting...
Second, it is easier to explain the heavy, low-tech-looking computers of CT in the framework of the T:NE setting than in the "golden age" Imperium. This works the same way as in the Battlestar Galactica remake from a decade or so ago: people have reverted to much lower level of electronics/information technology in order to make it difficult for AI enemies to infect ships. So you have compartmentalized computers and maybe even vacuum tubes and bulky primitive computers on ships, and bulky TL11 hand computers, working in such a primitive way to resist the Virus; any modern computer system would be a prime target for infection. Note that in my take on T:NE, the old Imperium had perfectly modern computers, and the bulky Book 2 computers are a new product of the New Era...
Third, Book 2 (and The Traveller Book) are small-ship rulesets. The big Imperium depicted in later CT products, as well as in MegaTraveller (MT), has very big ships. T:NE, on the other hand, reverts to a small-ship universe, as all, or most, big ships were destroyed in the Rebellion and Collapse - or worse. So, once again, a 1,200 ship is a battlecruiser and a 300 ton frigate matters in actual "big" combat. So, again, Classic Traveller and its "proto-Traveller" assumptions make perfect sense in the T:NE setting...
So how would I approach a hypothetical T:NE game using CT rules? I think that the best approach would not be using the Reformation Coalition, though this is a good option, but rather roll up a new two-subsector Traveller sandbox, using the general T:NE setting ideas but using or ignoring Traveller canon as it fits the setting; I think this will work better for CT to roll up two new subsectors rather than "convert" official subsectors into the New Era using the T:NE or the T:NE: 1248 conversion rules. The rest would be constructing the specific setting as desired... And using all the good bits from T:NE (such as the detailed government ratings from Path of Tears).
So how would I approach a hypothetical T:NE game using CT rules? I think that the best approach would not be using the Reformation Coalition, though this is a good option, but rather roll up a new two-subsector Traveller sandbox, using the general T:NE setting ideas but using or ignoring Traveller canon as it fits the setting; I think this will work better for CT to roll up two new subsectors rather than "convert" official subsectors into the New Era using the T:NE or the T:NE: 1248 conversion rules. The rest would be constructing the specific setting as desired... And using all the good bits from T:NE (such as the detailed government ratings from Path of Tears).