Congratz and good luck!
I’m not nearly the foremost authority on 3D or OpenGL, as I just started working with it about a month ago. However, the learning curve is remarkable. So I have faith that you’ll get far with whatever you endeavour.
To give my insight into your question though, I suppose that the question you ask is rather vague. You have too look at it from the standpoint of “What is my engine going to be used for?”
Once you answer this, you can start defining what features you want and what you need to get the job done. This is the approach that I am taking with the project that I’m working on. I’m interested in high detail, high polygon count, but at the expence of speed. I’m working on an inductrial app.
If I were to create a gaming engine, I would be interested in speed over detail and polygon count, but to a certain extent. The questions that need to be asked are :
Whats my target hardware (speed)
Whats my required features (lighting, fog, etc)
What kind of interface do I need.
The list goes on and on, but once you begin building it, you can start to flesh out an idea of what you’ll need to work on.
I hope this helps.