I'll go out and say something completely random which even I wouldn't buy but it sounds cool on paper; lego Thundercats!
In all seriousness, I'm with the Doctor Who crowd. I don't understand why the BBC decided to go with a CLONE brand! Inferior plastic! Horrendous designs! I'd take a minifigure Matt Smith over the real one any day! And imagine how awesome the Cybermen could look? The 'cyber-conversion' chamber could be designed in the same way as how you'd switch Lupin with Were-Lupin in the Shrieking Shack set. *sigh* BBC I am disappoint!
But I'd also love a Lego Portal line. I'm pretty sure Lego would have a field day with cake and cores, and all they have to do is 'censor' the neurotoxin. Not sure quite how we'd get the turrets past them though. But even so, Lego should grow up just a little. Just, a little. Say what you will about them 'not wanting to make violent stuff' but regardless of censoring, they've still covered Harry Potter, LOTR, Star Wars and Indiana Jones. And there's even violence in their own themes, like Alien Conquest (laser guns but still guns!) or Orient Adventures (I miss that series, nuts to Indiana! I want Johnny Thunder back!). Granted the latter was goofy since it was an IJ spoof but even so the threat still exists. How about castle? You're fighting the undead army of skeletons, that's pretty dark! And let's not touch Bionicle on that matter.
So...yeah, I'd be more than happy to see Lego Portal, Lego DW (though it's unlikely the BBC should cancel their current contract if they have any sense!) and some video game titles. Say what you will about them being a 'niche' market, but Video Games are a Big and Serious Business regardless, and I'd go nuts for a Lego Chain Chomp Wash or a Lego Zelda. Even better if it was Lego Sonic. The minifigures don't have to be a mess - look at Chima. Besides, I've been using my ten Chaos Emeralds for years now and they need someone diabolical other than my Time Lord to steal them.