My ending is weak

At least that's the feedback, I got from one of my readers. I am now taking an ending poll. It is essential to get honest feedback, not just a general pat on the back you wrote a novel--good on ya mate--kind of thing.

The problem with my ending according to this reader has to do with what I've planned for the next book in the series. She is one of the only people who knows what that is, and so I can kind of see her point, but at the same time I like it.

My problem with my ending is that I don't know if I've wrapped up everything as well as I should. But I'm not worried about my ending overly much until I rewrite the beginning and middle. Two little relationship changes between the main character and two other characters call for major rewrites, a whole new family dynamic, and hopefully--I think it will be--a stronger novel.

My question is how to balance developing the characters and fleshing them out without throwing off the pacing of the novel. I think my pacing is pretty good. People can't put down the novel by the time they reach page 80. I know I'm working on the first 80 pages to make it hard to put down from the beginning, but I need to flesh out the characters and build the relationships. 

It will come. I know it. Any advice or tips welcome!