The current multiclass rules suggest that we trade an at-will. My question is, does gaining an additional at-will instead of trading one seriously affect game balance? The basic power currency is encounter and dailies which are of limited use. At-wills are technically interchangeable. Is the versatility not a reasonable benefit considering the loss of feats?
Personally, I don't think it will unbalance your game at all. I mean, they already had to spend one feat getting the multiclass option, now they're spending another, and due to the economy of actions and relative power of all the at-wills, it's extremely unlikely to cause any noticeable issues.
The only real problem is that this is arguably better than taking any normal Feats, because Feats in 4E give you so very little, generally. I'm not sure that actually holds water, though. In a highly-optimized party, where every Feat is going to good use, I don't think that many people would actually go for this. However, in a party where people are less optimized, I think you might find that everyone wanted to be multiclassed.
Again, that's not likely to cause real balance issues (4E is more stable than that), but it will change the flavour of 4E, and cause more "crossing over" between roles. For some people, that will be annoying, for others it will be exactly what they want.
Edit - Obviously, we're talking about at-wills here. With Encounter and Daily powers, because they're balanced differently, having more of them wouldn't just give more options, but rather just be better, so don't extend the idea to those.