Well, assuming that the GH AT-AT actually does get released next year the Hoth sequence from ESB is actually one of the more "fleshed out" environments from the OT films in Galactic Heroes format (I don't collect PT/CW Galactic Heroes, so I can't speak for those).
In the area of vehicles and beasts/large figures, we've already got:
AT-AT (keeping fingers crossed for 2010)
So that would just leave the Scout Walker and Rebel Transport as far as vehicles that were actually visible on Hoth. I think the Transport would be cool to double as a carrying case (just like the vintage toy). However, neither of these would be considered absolutely critical to the Hoth scene.
That would just leave large accessories and playsets/environmental set pieces:
Radar Laser Cannon
Vehicle Maintenance Energizer
Tripod Cannon (they could include a special reposed Snowtrooper with it)
Hoth Trench (done in the same style as the vintage Imperial Attack Base)
Ion Cannon playset
Shield Generator playset (with exploding shield action)
Medical Chamber with Bacta Tank and Luke in a diaper
Since this line is more of a true, kids toy line, most of these playsets could be combinations of molded plastic parts with cardboard backdrops (in the same style as the original Kenner playsets).
Many of those accessories could be made fairly cheaply and released in "troop builder" battle packs. For instance, make the Tripod Cannon with a special Snowtrooper, and maybe 3 or 4 of the current Snowtrooper. Throw in a Wampa and Threepio and you got yourselves a battle pack (most people will know what cutscene I'm talking about here).
Same with the Turret and Radar Cannons. The Tripod Cannon doesn't need to be huge, but preferably not made of cardboard. Include the Radar Cannon and maybe a few different variations of the Hoth Rebel Trooper and Hasbro would have a pretty decent little "Hoth Trench" BP.
OR the Tripod Cannon, Radar Laser Cannon, and Vehicle Maintenance Energizer (which really should have been included in the Snowspeeder BP) could just be included in the standard 2-pack figure line (similar to the Anakin with STAP set).
As for figures, well there aren't really that many left to do:
They could get specific and make characters like:
Snowspeeder Pilot Wedge
Snowspeeder Pilot Jansen
Snowspeeder Pilot Zev
Snowspeeder Pilot Hobbie
assorted Astromechs, Power Droids, etc.
But I'm not sure how far they would get with those (maybe a future rerelease of the Snowspeeder BP could include Wedge and Jansen, instead of Luke and Dak). Most of the figures from this list have "battle pack filler" written all over them, though.
Anyways, just some thoughts about the Hoth sequence after combining my Snowspeeder BP with the previous Battle of Hoth BP.