I read "Concrete Jungle" and agree that would've been a much better movie.For Predator 2, I say go with the Concrete Jungle story from Dark Horse Comics but I don't know who you cast as Dutch's Brother.
As for the Starship Trooper sequel, First improve the first movie,
1 have the Power Armor, if not for all soldiers in the movies ,than at least one or two per Squad as they did in the Roughneck TV show.
2 Treat it like a War Movie instead of a episode of Beverly Hill 90201
3 Better casting, Get rid of Denise Richards and Neil Patrick Harris for a start.
4 Get a Director who enjoy the book. Paul Verhoeven has said that he could not make it past the first two chapters in the Novel.
5 Another writer rather then Edward Neumeier . How about getting Joe Haldeman to do the script?
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I liked "Starship Troopers" pretty much the way it was and wouldn't change anything but I never read the book but if I had, I might feel differently about it.