I don't think upping the damage scale is too big problem. I think the real problem is doubling the radius of pbaoes and cones. I'm sure others can relate to this but AoE is the reason I love to play melee. I play a spines/elec brute with double damage auras and the more crunch and ripping and echoing sounds I hear the more fun it is. If I'm going to be short-changed out of that experience by only having half the radius and lower target caps, well, it might be worth re-rolling. I wouldn't really care about what damage loss I take, I'd be too amazed at how far my spine burst, quills, etc are reaching. The more enemies I can take on and hit at once, the better. And as many others have mentioned already, hitting more targets = more dps. When a tank can hit 16 targets with his pbaoe and I can only hit 10, he's contributing more to getting everything dead than I am.
I get wanting to increase the radius and target caps of pbaoes and cones to be able to reach more targets and keep the attention easier, but scrappers, brutes and stalkers and other melee are definitely going to feel short-changed for sure. It feels like the problem you're trying to fix here is already being taken care of by the new changes to gauntlet. A 15ft AoE taunt with every attack sounds like a great way to keep the attention on the battlefield and make the tanker feel more in control, imo. In fact I would consider raising the cap on it to 10 instead of 5?
Also, this might be a dumb suggestion but have we ever considered just bumping the threat level of tanks to 5.0 (maybe even higher) and keeping brutes at 4.0? Sometimes when I'm on my brute I find myself peeling bosses/AVs off a tank just by trying to dps. I don't even use the taunt power (but I do have debuff procs which I believe contribute).