I actually hope the whole Kite backstory is kept as part of the TV series, it would suck if say someone saw the series but not this new movie and thus didn't know who Kite was. I see anime movies for Shonen series as "extra" material basically, or optional. So whatever its about should either be a very small side story or completely filler, not something integral to the plot like Kite's character.
acctually a kaito thing may not be a bad idea. Of course I'm not saying put in his intro and past with gon in a film, that would mots certianly stink, but what I'm saying is if we did one on KAITOS's acctual past. It could be like a flash back and forth sort of thing where kaito challenges gon to hunt for something/one that related to his past, and all the while kaito's observing him (and most likely killua) we'd get flashbacks on his past. dip in a confrontation between him and hisoka and we may have a pretty cool film.
What would stink is that it would once again not include kurapika nor leorio, but hey its a thought. one I would hope dosn't completely come true, but maybe as a second film or something.