TOWNE32 came up with an excellent idea over on the REVIEW thread.
(http://originaltrilogy.com/topic/The-Force-Awakens-Official-Review-Thread-SPOILERS/id/46786/page/18#888866)

Remove the destruction of the Republic homeworld to the final battle at the end. Essentially this would be a much more important target than the Resistance base - so the stakes and rushed plan would still be important. The Resistance’s failure would provide a much more bittersweet ending.

If you wonder why the republic fleet isn’t involved - perhaps they are too far away to arrive in time - or otherwise engaged.

Perhaps the destruction of Starkiller itself might be as a result of its use. Maybe it is designed for just one shot - so the convenient destruction by X-wings could be avoided completely.

I wonder how essential the planetary shield plotline is? Could they just be undertaking a rescue mission? Chrome head could be abducted to just find out where Rey is - only for the heroes to find out she has escaped.