12 abilities from one skill: no, that's not going to happen, for two reasons. First, that would rob a lot of the strategic elements from the skills. Right now powers like Decapitate are a hard choice -- it does a ton of damage but a very slow cool down, and the number of slots it can go in is very small, so you have to decide whether it makes sense for a particular outing. Sometimes it does, sometimes it doesn't. That makes the choice interesting, but balance able.
If you upped the number of slots, it's a no brainer: put Decapitate on the bar and use it whenever it comes up -- it's nearly the highest damager in the game with a pretty reasonable Power cost, so if you've got a ton of Sword-boosting gear on anyway, why wouldn't you always include it? Any strategy is lost.
Second, that would teach you that the best way to play is to focus on one skill. Focusing on one skill makes it much easier to gear up, and if you had 12 slots for that skill, you could have pretty much all the toys of that skill simultaneously. But I REALLY don't want players to get into the habit of thinking they can survive with one skill. You will need both skills. As a very trivial example, there will be dungeons where there are monsters immune to fire and ice, for instance, and if you've only got Fire Magic at high level (and geared well), you won't be able to play it. That wouldn't be YOUR fault -- that would be MY fault for teaching you that you can be successful with only one skill. (It's often true right now, in the low levels, but these are the newb levels, remember.) So I try hard to push players to understand they need to level at least two combat skills. I need to be able to throw all kinds of scenarios at you and know that you'll be able to adapt to them. If you only have one skill, you wouldn't always be able to do that. Which restricts what kind of experiences I can throw at people.
6 abilities per skill but spread arbitrarily across two bars: that doesn't present balance problems, but it does make it very hard to explain to newbies what's going on. It may be a good advanced GUI mode down the road, though.