I do agree they have to be careful. The last few movies did the the series in...don't get me wrong, I personally enjoyed them just because they were Trek, but they failed miserably. Enterprise never took off I thought, it never quite had the following...you might blame the fact that it was on UPN and that had something to do with it, but it was well written, just never caught.
IMO Enterprise failed for the following reasons:
1. The theme song wasn't appropriate
2. They tried too hard to invoke sex in it that it didn't seem natural
3. Diehard fans whined too much that it didn't stick to original story line
4. It came out in an era where reality tv shows dominate
5. The acting in the first season was terrible - almost as bad as many of the non Trek Sci Fi shows currently out there
The 3rd season was absolutely brilliant. Than half way into the 4th season when they pulled the plug, the show got sloppy and had the WORST ending to any star trek series in the history of the franchise. As a Star Trek fan, I was embarrassed and ashamed of that episode and the associated cameo actors Riker/Troi. Pathetic.
I hope they don't bring in old actors in the new movie just to get people to come and see it. It has to be natural or it won't work.
About the new movie
The way they can pull the new movie off would be for Spock to reminisce about the past and narrate the story about the period between the academy days and the 5yr mission. Though I don't think that will happen since I don't believe Kirk ever met Spock until TOS. I guess we will have to wait and see.
My Idea for a new series:
A Star Trek show from the Klingon perspective. Think of how cool that would be since most diehard trekkies favorite characters are Klingon. There is so much potential for new story arcs if you did them outside of the human perspective.