There are a few problems with the idea of making blade flurry the mastery ability. 1: It is an active ability, and I don't think any spec for any class has an active mastery. 2: Changing blade flurry into a mastery would remove combat's level 10 default active ability, which I believe every class and spec has. 3: Blade flurry only has a 30 sec cd. I don't understand how you don't find that to be short enough, especially with the 15 sec duration. If we wanted it can be up 50% of a fight. Another downside to a longer duration is the fact that we have reduced energy regen while it's up. There are very few boss fights where I would want blade flurry up 100% of the time. Come to think of it, Valk's in ToC is probably the only fight where 100% uptime would yield positive results. Aside from that, 30 sec cd and 15 sec duration is plenty long and I find myself actually clicking the buff off about half the time I use it on boss fights with close adds.
I do find something positive in your post, however. I like the idea of bringing HaS back. Though an easier and effective fix from Blizzard, as stated previously in these comments, would be to reverse the effects of MG while keeping the percentage values.
Overall, combat needs alot of twinking by Blizzard before it's viable again, which is sad. I have tried all three rogue specs in Wrath. (yes I tried sub just for fun regardless of how bad EJ said it sucked, which they were right) I leveled combat, started combat at 80, and loved combat. I didn't switch to mut until I started reading into my class and learning to play better. It was at that time that mut was ahead of combat, but I switched right back to combat without hesitation as soon as combat was on par with mut. I then vowed never to stray from combat again, as It was my favorite pve spec. It is just sadening that combat is so "meh" right now, and I'll be playing another class before I am forced to switch to a viable spec I don't like to stay on top of the charts.
Sorry about the long post but thanks for reading. It's hard to express an opinion on somthing like this without going so into detail.