First, I commend your desire to write a full-length novel. That will require a great deal of discipline and time.
As to suggestions, I would simply suggest that you write what interests YOU. If you are not passionate about the characters, plot, setting, etc., you will find it difficult to write a long story, much less a novel.
Personally, I like any era of Star Trek, whether using canon characters or not. When I'm reading, I'm looking for stories with:
- Interesting characters / well-written dialogue
- A story-line that flows without being predictable
- Good imagery but with limited techno-babble
- Proper grammar / spelling / etc.
As an observation there have not been too many stories written from the TOS era, so you might consider that when planning. Good luck!