TNG needs new FX and that's really about it.
No, that's not even close to it.
TOS episodes existed on film. All that was needed was to transfer the episodes to HD and to clean things up a bit.
TNG episodes do not exist on film.
They only exist on video tape and the quality of the DVDs cannot be improved on. The way it looks now is the best it can ever look.
The only way to make TNG HD is to recreate every episode from raw film footage. The film still exists, but the film would have to be edited together again from scratch into a completed episode and new FX added. It's not, strictly speaking, going to be a remaster. The completed episodes that exist now can't be remastered in HD.
TOS only had to be transferred again. TNG will have to go through seven years worth of post-production. A much much
greater challenge than remastering TOS to HD. How long will that take? Well, it took seven years the first time. (Then again, this time around the audio, music, and sound FX are basically done and won't require the work involved the first time through post-production. Still, make no mistake, this project is going to be much much more labor intensive than remastering TOS was.)