I think it's certainly reasonable to include a Gelagrub in a Felucia Battle Pack if Hasbro can fudge the width of the box a bit. The SITS box was slightly deeper than the mass retail BPs, so since this wouldn't be an item that would ship alongside anything other than itself, I can't see why a slightly larger box to accommodate the girth of the Gelagrub wouldn't be possible.
The only problem would be getting Hasbro to concentrate more on the actual toy than the presentation of said toy. I'd much rather have Bly, two Evolutions Felucia Clones, and a Gelagrub with rider, but to do that, Hasbro wouldn't be able to do their flattened movie scene approach with the presentation.
As I said earlier in the thread, Hasbro could do a kitbashed rider for the Gelagrub by using an AT-TE Tank Gunner body (due to the ball-jointed hips) with a soft goods kama. That said, they probably won't think of that themselves.
Regardless, I can't think of anything, action figure wise, outside of Bly, multiple Evolutions Felucia Clones, and/or Secura that would be included in this set and not end up sucking horribly.