Predator
Gremlins (not a trilogy tho)

Grindhouse trilogy?
Death Proof, Planet Terror ,
Machete

Another not-a-trilogy, but at least it's already practically done: The Bible

Evil Dead of course. Tiny Lego Ash... Heh.

'Dawn of the Dead' and 'Day of the Dead'

• Some Lego Zombie games. The series for 28 Days later, or Night of th Living Dead, or Walking Dead. Walking Dead would be nice if they want to get into episodic DLC. Adding more content & characters as they expand the game.
• Game of Thrones (Song of Ice and Fire) Lego series. I'd love to see their take on it.

back to the future

Die Hard, Alien, Scream, Terminator aren't trilogies anymore, and I believe the movie count is actually 5 for Die Hard, 5 for Alien (counting Prometheus and not the 2 Alien vs Predator which aren't canon), 4 for Scream and 5 for Terminator (counting Terminator Salvation: The Machinima Series) now. But good Lego games? Possibly. I also can't see Home Alone working as a Lego game, but I'd see it before Austin Powers.

This also isn't a trilogy, but I think Underworld could be cool as a Lego game. I can't say the new Batman trilogy since, well, Batman's been done by Lego (very fun game too).

Teenage Mutant Ninja Turtles.

The Vengeance Trilogy

• LEGO Minecraft would be an interesting idea, although it would probably turn out as a shoddy ripoff/remake that is unnecessary.
• LEGO Godfather sounds pretty meh, but hey, they made 2 games out of the series so far so who knows
• LEGO Samurai Jack... My goodness, I could pay fortunes for that.
• LEGO Jackass. Okay, just kidding, this is a fucking horrible idea.
• LEGO Mummy, although I wouldn't care if they disregarded the last movie. A throwback to the classic Karloff one would be more welcome than that.
• LEGO Avatar, duh
• LEGO Riddick, what with the newest movie showing signs of becoming a possibility
• LEGO Firefly+Serenity
• LEGOlas, The Game

Yeah, I'd take most of these as non-LEGO any day tbh, but hey, I enjoyed me some LEGO titles in the past so I'd be up for those too.

Evil Dead and Army of Darkness. lol Though they wouldn't be family friendly, I'd love to see 'em Legofied. ^-^

LEGO Bad Rats

The Land Before Time

LEGO Hellraiser

A number of those would be better without the LEGO license. I, for one, can only take so much LEGO-izing in one go.

LEGO Deadpool, Transformers, Borderlands, Gears of War, Halo (inb4 megablocks qq)

Not meeting the movie trilogy condition as well, but how about some LEGO / videogame saga crossovers?

• LEGO Need For Speed would get us some nifty car models and city street tiles
• LEGO Mario and Luigi, the transition from paper world to brick world
• LEGO Bastion, the bricks spawn out of nowhere!
• LEGO Portal, the levels were quite blocky to begin with and GLaDOS with a NXT as a heart? I am sold.
• LEGO FIFA/Pro Evolution Soccer. To hell with realism, I prefer minifig teams!