Assuming a radical revision on the appearance isn't what you're after, I think the biggest obstacle is that the original sculpts don't lend themselves to modification. Specifically thinking of the Gorgon here, where I prefer both the 2 and 3 variants over the base model. Pulling off the LRM-10 would be no small task, and I'd be inclined to resculpt over modify for the amount of work that would go in and the uncertainty of what would be left of the torso. Then it's a question of cost versus sales to justify it. Protos clearly aren't everyone's cup of tea, and trying for a subset of a subset isn't going to generate much buzz.