I doubt we'll ever see plastic kits, since Halo's target demographic really isn't much into modeling these days. But I could be wrong. I certainly hope so. I've never scratchbuilt anything, but I model mostly armor and I've wanted to scratch a scorpion for a long time now. Just not sure how to go about it.
Also, Lego doesn't have anything to do with Halo. That would be MegaBloks, which are a cheap (in every sense of the word) imitation of Lego.