one of my players came up with the idea of playing a fighter who will be impressed by the furiosity of barbarians.
My advice:
First off: can the multiclassing wait until March? If so, you're fine, since we'll get MC feats for the Barbarian in the PHBII (at least, I'd expect as much). Perhaps the fighter is impressed by them now, but takes a while to learn their fighting techniques. Like a level or two in game and a month or two in real life.
If you can't wait, you should consider if the being impressed by barbarian ferocity even needs mechanical representation. There are a lot of things that make up a character that aren't on their sheet. And since "barbarian" the rules level concept is different from "barbarian" the character level concept, a fighter could already be a "barbarian" to the other characters.
Thirdly, could you just take some appropriate fighter options that produce a similar sort of character. The aforementioned Battelrager is good for this, as are other Fighter powers that grant enemies attacks or Combat Advantage. Or Stances, which are already quite a bit like Rages anyway. These fighter options might be retrained into Barbarian stuff once the PHBII comes out.
Fourthly, you could houserule a Barbarian MC feat to use for the next two months. Since it's a short time frame, you wouldn't need to be super concerned with balance: just pick a class feature, downgrade its frequency of use one step (At Will to Encounter, Encounter to Daily) and training in a Barbarian skill. Then when PHBII comes out, swap it for the official MC feat (or possibly keep your own if GM and player(s) all think it's better).