Endings - The Novel's promise

I am at the end of my work in progress again, and still struggling for the perfect ending. I think I have probably written another book's worth of endings! In desperation I asked two other novelist friends who had read the beginning chapters to predict the ending.
The results were massively illuminating, not least because they had completely different ideas about a possible ending.

When I began the book, the ending was the only clear image in my mind, and it drove the book, but now that I come to write it, I find the characters just don't sit happily in it.

When I asked my two readers for ideas it was fascinating to see their expectations for the end of the book - each wanted a different character to die at the end of it. Each had clearly identified with a different character. Minor characters made a sinister reappearance, making me think that perhaps they lived a bit more in the readers memory than they should. Their views as to what might happen in the ending have however freed me from the tyranny of my own view, and opened up further interesting avenues to explore.

One of the readers suggested that the difficulty I was having with the ending was because in a novel where everyone is a criminal, the notion of characters getting their "just desserts"  simply won't work unless you want mass carnage, and besides which we have grown to know and love them all.

"Of course!" I mentally shouted, "That's just it!" He'd nailed my dilemma, which up to that point I could not put into words. So I can't kill off my murdering thieving characters, but I can't exactly let them off. To save them all in a sort of  mass leniency does not seem such a good idea either, making it just too sentimental.

I have Nancy Kress's Book, "Beginnings, Middles and Ends" where she sensibly suggests the writer should fulfil the implicit promise (s)he makes at the beginning of the book - hence my rather late-in-the-day attempt to find out from my reading guinea pigs what that promise seems to be. Turns out of course it is not quite that simple, as both perceive a slightly different promise.

There is of course the option of re-writing the beginning to fit my original ending, but 125000 words in, I don't think I'll be going for that option! So I shall continue to roam round these new suggestions until I hit the satisfying "Yes!" that means I have finally found the one that works.

If anyone else has top tips for generating the perfect ending, it would be good to hear them.