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If they keep being successful I don't see while they would stop at only three. The only suggestion I'd have is not to let Joel Schumacher anywhere near the franchise.
But it wouldn't be a trilogy in the sense of one continuous story anyway. Of course like the comic books, the stories are purposefully and inherently episodic.
Down the road if they want to get a bit darker, I'd love to see an adaptation of one of my favorite stories from the comic books: the Kraven's Last Hunt stroyline. It's like Spidey's Dark Knight Returns. Well, not THAT good, but a very well-written and dark story. Waaaaay too dark for what I'm sure they hope to keep a kid-friendly franchise (no Batman Returns missteps), but a terrific story just the same.
But part of what made that time period in the Spidey chronology so cool for me, at least graphically, was it was part of the black costume phase. That could be a fun thing to examine on the big screen, but since reproducing Secret Wars is extremely unlikely - damn near impossible, they'd have to find another way for the suit to be found. And the whole space alien thing may be too "comic booky" anyway. But that could all eventually lead to Venom, and that would be a terrific villain, and one that could be easily realized thanks to CGI advancements.
Whoops! I think my geekiness is showing - just a bit. Can you tell I used to collect comic books like I now collect DVDs/LDs, and can you tell what years this took place?!? Geesh, what a frippin' nerd I is.
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"Film is a disease. When it infects your bloodstream it takes over as the number one hormone. It bosses the enzymes, directs the pineal gland, plays Iago to your psyche. As with heroin, the antidote to Film is more Film." - Frank Capra