I also want them to extend the episodes. If they have to basically redo the entire post production process it shouldn't be a big deal to insert scenes that were cut for time or extend battle scenes. Who cares if the episode suddenly runs 4 minutes longer, it doesn't have to fit into a timeslot. And if they decide to air the episodes in syndication, they have to cut out footage anyway, so what's the difference.
Basically, what I don't want is a faithful recreation of the original episodes in HD. They can use multiple angles and seamless branching to include the original versions on the Blu-Rays, even if that means no HD for the space scenes.
The only problem with extending the episodes is that all of that CG costs money, and I doubt they'll have a monster budget for this. I'd rather than spend the money to make the existing shots look great, rather than spread themselves thin trying to add more. Although it would be nice of them to add a bit more where the original couldn't due to technology limitations. When I was watching TNG for the first time only a year or two ago, I would call out "GENERIC ESTABLISHING SHOT B! (or whatever)" every time I saw those same 3 shots used over and over.
It's sad to hear that they don't have the original film. It would suck if TOS was able to have a good high def transfer from film, but not TNG. I guess my lifelong dream of seeing Picard facepalm in HD won't come to pass