I'd say you could do it with any list that lets you take a large number of light vehicles or walkers and not so fantastic troops. I think either Orks or
IG are going to be your best bet, but there are some other possibilities.
On the Ork side you can easily do mini tanks as kanz, slightly bigger 'heavy' tanks as dreads, and have a pair of Big Meks so you can still pack 9 kanz and 2 dreads into a list. You can also easily model most speed freek vehicles as gretchin crewed equivilents; Deffkoptas, buggies, skorchas, etc. You could probably even get away with bikes with one gretchin driver and a couple more on a fighting platforms on the back. For troops, you have regular grotz and deffdreads. This is by far the easiest way to do the list.
For
IG, you have Sentinels as your main mini-tank substitute, though again, the larger vehicles could be Gretchin 'battle wagons'; Hellhounds, Chimeras, and Russes. As an additional bonus, grotz make much better substitutes for
IG, statwise, than for anything else. You can pack 12 sentinels (9 in fast attack, 3 as an attached
HQ squad) and 3 Leman Russes into a current
IG list without difficulty, and then add more chimeras as battle wagons with crew for troops. You'll definitely have a more troop model heavy list with this than the ork or other list possibilities I can think of. Fortunately, the new gretchin box is really cheap (if you've got some spare heavy weapons around, $15 for an entire
IG troop unit!). The biggest downside here is that with a new codex coming, it's hard to guess what you'll be able to keep playing and what will change.
Those two are definitely the obvious ones, so now we're getting into the more esoteric options. These ones you'd have a real tough time justifying at a major tourney, but would probably be just fine for store tourneys and friendly games. You'll have to be a little creative with some things. For example anything that's "hover" (Tau tanks and suits, Eldar tank, bikes, or vypers, or Marine landspeeders), deffkopta style helicopters would be good alternatives. And you get bonus points for Final Fantasy-esque heli-carrier platforms with multiple rotors and/or zeppelins on any of the larger tank types.
Again, keep in mind that you want lists that give you lots of light vehicles (or equivilent) and have at least one viable troop that you can sub gretchin in for.
If you're willing to get away from the pure armored company aspect, you could go for a more Air Cavalry oriented approach. Break out your copy of "Ride of the Valkyries" with a little
Aporkalypse Now theme, because Beakies don't surf. Model up vehicles and suits as helipcopter transports and gunships. You can take upto 15 Piranha and/or 15 crisis suits as lighter attack koptas, and another 3 Hammerheads and Skyray as heavier ones. Broadsides work as heavy kannon or zzap-gun armed tanks. Grotz as Tau Firewarriors is fairly believable, especially if you stick to pulse carbines instead of pulse rifles, to keep their range more orky; Devilfish as kopta-transports are obviously mandatory in this sort of list. The more your vehicles look like ork-ified versions of Vietnam-era Huey and Cobra helicopters, the better.
Eldar is another somewhat odd possibility. You can do the same thing you did with Tau, opting out of "tanks" and going for a more Air Cav theme, with Waveserpents as transport Koptas and Falcons and Fireprisms as heavier gunships and Vypers and jetbikes as light attack koptas. If you want a more ground oriented force, Warwalkers make good mini tanks, and you can take 9. Wraithlords can be heavier tanks or deffdreads, though they compete for slots with your warwalkers. The Aspect troops don't match up real well, but Wraithguard could be represented by mega-armored gretchin carrying zzap-guns or similar in your elite slots. You've got a nice troop selection in that gretchin work fine as Guardians or the aforementioned Wraithguard in sufficient quantity.
HQ is probably the hardest slot to fill with pure gretchins so you can get by with either some Orky Weirdboyz as Farseers or a mini-stompa/dread substituting for the Avatar.
Marines seem an easy match at first, as you could have your mini tanks count as dreadnoughts of various stripes. You can get 6 of those in a list if you take a Master of the Forge (easily modelled as a Big Mek, preferably with Shokk Attack Gun for a Conversion beamer). Additionally, you've got landspeeders (heli-vehicles) and/or bikes (preferably scout bikes, using the 'gretchin combat platform' idea) as other vehicle platforms. The big downside here would be finding suitable troops,since single gretchin as as Space Marine Scouts is likely rather stretching it for most people. However, the other options are worse, so sticking with shotguns for the orky short range is probably the best possible choice if you wanted to go this route.
-D
