Halo is a fun enough video game, but it's just one in hundreds of other decent first-person shooters. I don't really see Halo as a particularly outstanding game, and there are plenty of others that I would rather see.
Of course, as an M rated, war-centric game, it would probably be against Lego's core ideals to make it anyway. Anything out of the range of Lego's core audience is pretty much out of the question. Even if an exception were to be made, I would still prefer something with more depth to it than Halo.
I don't mean to offend or anything, but Halo is a pretty generic sci-fi game by today's standards. Lego could easily make a sci-fi line like Halo without having to buy a license. Brand names are restrictive by nature, and this particular one would have all of the cons without the pros.
Put simply, a Halo brand would alienate more than it would attract, and even without the reasons listed above, I still don't even particularly care for Halo. In short, nope.