Of course it would all be very illegal, but after stumbling through my first steps of World of Warcraft (where everyone looks the dam same with the same clothes as you pick up armor), and wearing a tunic with candle drippings that I had to LOOT (not feeling too much like a hero for THAT), if there's anything that will take me back to my beloved game, just count me in.
No, it wouldn't necessarily have to be. There are a lot of factors making something illegal or not:
1) reverse engineering by itself is not illegal.
2) using knowledge gained by reverse engineering to create something new is not illegal if you don't break any patent/IP laws with it.
3) create something new that works similar to something existent or something no longer existent is not illegal if you don't break any patent/IP laws with it.
Now while 1) is very self explaining, I'll give you an example for 2)+3) from the Star Wars Galaxy EMU. The team needed to recode the login of the client to work with their servers' web adresses. They had to create a new log-in screen - the original screen played Star Wars music in the background. To not cross the Intellectual Property of Lucas, their download did not contain copyrighted music nor backgrounds of the original log-in screen, but screenshots/backgrounds and a music-loop of their own. The user however can easily replace this music-loop with original Star Wars music the user legally owns from other Star Wars games, his original SWG copy or any soundtrack (and naturally no one can do anything against the user deciding on a piece of music he does not legally own) as the musicfile can be directly accessed, renamed, replaced etc. Key is: the crew behind the EMU is not providing anything that's not their IP (the code of the EMU server in fact is their IP as well as the code of the client modification is their IP) and thus, the C&D has nothing to be based on. It's a thin line to do a ropewalk on, but it can work out.
Additionally, neither SOE nor LucasArts did care much to work against the EMU, as the EMU has limited itself to emulate the game at a point of before a certain update - so to a "service" the original companies no longer provided. (This gave them the chance to work on the EMU while the game was stilll running and that's basically why they have a running version already to answer a previous question.)
4) the definition of IP/copyright/trademark/patent violation greatly differs depending on where in this world you are.
If you place the server containing the downloads and the EMU service in Korea, China, Taiwan - no chance and your own fault. You will not be able to fight NC at their home turf in legal stuff. The US and UK could both be tricky, depending on how the EMU-code willl work in the end, what patents NC does still hold in the US at the time the EMU is ready etc.
In France or Germany however, you can avoid IP/trademark/etc problems by being to a certain degree "off" the original (I think it's about 20%, this can be looked up simple). Frauds from the eastern-asian market use this to flood fill the home market of both countries with product piracy. Some lawyers go the "but our I has a red dot over it!"-way in a fictional case of an original product "Hogi" turned into "Higo" by the asian pirates - and get away with it. Could you imagine how many judges would be amused to show a company from Korea "the finger" over a similar case in that two countries - especially when there's a non-profit group on the other end
And if for some reason Europe does not work out, you have Brazil, Angola, Uruguay - or Switzerland, which is Europe while not really being Europe, as they kept their neutrality and thus are completely on their own.
So don't be to sure about "being illegal" from the start. They don't auto-win because they did with Tabula Rasa. They don't are in the position of having the law on their side because they tell you so or wrote the EULA. The more of the IP needed to play CoH is already on your harddrive labeled "client files", the less likely NC will win on IP related laws....