Truthfully, there are two units that have drawn me to the codex: the Lone Wolf, and the Thunderwulf Cavalary.
There's also the chance to explore an old theme I like: the warrior-mystic. I'm tempted to shoe-horn Clan Nova Cat (of the Battletech universe) into a marine chapter. They have the same kind of feel, for me, that the Wolves have. Plus, I think base white with black and gold accents can work out.
The Lone Wolf
I don't honestly think I'll be hard-pressed to model a Lone Wolf. I'm looking at Terminator Armor, a Storm Shield, and a Chainfist. Why?
T4, Feel No Pain, Eternal Warrior, two wounds. You need 24 regular wounds to break 2+ armor with Feel No Pain. You still need 6 wounds from weapons like lascannons/melta/plasma/power weapons that ignore your armor save. Are we durable? Potentially, we eat lots of fire.
If not, a Chainfist combined with Beast Slayer (re-roll hits against stuff with T5 or higher, and maybe against vehicles?) and the storm shield means he can still ruin days. Hey, Mr. Regular Dreadnought, there's a better-than-even chance a Lone Wolf will be around to smack you with a Chainfist. It's the ability to threaten any kind of game.
The Thunderwulf Cavalary
...where is the model? I'm looking at running 6-9 of the things, potentially. And...there is no model. There is Canis Wolfborn, a $41.25 kit with about zero conversion potential.
I'm not spending that much and doing that kind of work to run freaking thunderwulf cav. No, I'm going to do it cheaper, and that means conversions. I'd also like to maybe do it as large cats. Thing is, what'll I use as a base?
I'd be willing to go mecha-cats (or partially bionic'ed up) for it, so long as they're suitably imperial-looking, which is to say that they're not so smooth. I'm sure there are also other ways to represent it, like some kind of advanced exoskeleten or some such. (I'm thinking something analogous to a marine in ESWAT armor or an armed suit ). I mean, is it so hard to believe that some marines might model their wargear after a predator, or something?
There just aren't a lot of candidates for conversion, even, from the GW range. The Bloodcrusher/Juggernaut models have the right dimensions, but there's the issue of modding metal, and of course the cost: $30 a head. Yeah, that's a little bit more. Most of the cavalary-type models in fantasy have the right price, but are a bit weedy.
I guess if you wanted a more bug-like theme, you could try to bash something out of spider riders or Necron Destroyer bodies if you can bits-order them.
If you wanted more saurian, you could maybe try to use Tyranid Warrior bits and a healthy dose of Green Stuff, and some new heads, and/or Lizardmen parts, but still...the only really cool Lizardman model is some dude name Kroq-Gar on a Carnosaur, and while that would be tight, it's also a $50 metal model.
the other real option I see is finding an appropriately-scaled toy creature, and then bitting it out as best you can.
Anyone else got any ideas? Frankly, I'm a bit disappointed GW just utterly failed to release a model for one of the few things that the Wolves have that REALLY sets them apart.
EDIT: Looks like Stelek crafted some, and here is a picture. Chaos Warriors + Beast of Chaos. Not bad. I just want mine bigger, so I'm probably going to cannabilize my half-assed attempt at Bloodcrushers. Already got 15 of the things, more or less. Might as well get some use out of them.