I think I would like that. Older characters, aimed at an older audience, longer books. But at the core, they would have to stay the same. The issues the characters dealt with, maybe a little refinement more development and diversity, would have to be about the same.
The sense of humor of the books was what really got me hooked (twice) so that would definitely have to stay.
The shades of grey in the series, that you aren't really that different from your enemy, when is killing ethical, when is it not, what's justified in a war for your very survival, how you can fight without becoming the very thing you're fighting against, that would definitely have to stay.
I'm not really against more romantic involvement between the characters, but I do not want to see the series become about that, I hate it when that takes over the plot of stories.
Maybe some more moments like in the books 20-22 where there's this utter desperation, and exhaustion, where it's not about a quick battle but a slow methodical chess game. Although again these moments shouldn't completely take over the series it would be really cool if there were more of them.
Times when people pay the price for making the morally correct decision. I mean, I'm hard pressed to think of a time when it bit them in the butt for taking the moral high ground despite that they did it a lot.
Less avoidance of killing in the series, most of the time the books went out of their way to mention that they didn't kill some one in a fight. I absolutely do not want to see violence and killing just for the sake of violence and killing, but I also don't want to see it avoided just for the sake of it being avoided. If it fits the story then it should happen. The avoidance of it also waters down some of the ethical and emotional dilemas the characters face.