I've gotten some feedback on some of the other places I've posted this, and I wanted to come back and address it here, too (everything's paraphrased by me):
I agree that Blood should not struggle with AoE threat, but I don't agree that we should rely so heavily on JUST Blood Boil. With my suggested change, while you would lose some of the threat generation you had per cast of Blood Boil and Heart Strike, you should be able to spend more runes on Heart Strike, thus counteracting any lost threat. Additionally, I believe that Blood burst AoE damage is out of line and needs to be nerfed, and the main contributor to that is Blood Boil.Blood Boil being strong is necessary for our AoE threat, and even more so with your suggested change to reduce the number of targets hit by Heart Strike.
This change is more focused on making Bone Shield easier to keep up - but still mandatory - so that the annoying "spam this button" routine that we've undergone when Blood Boil (and now Marrowrend) was our filler ability can fully die. This change seeks to use the interesting and involving DnD-Cleave mechanic on a core defensive ability so that we can get rid of the anti-synergy Catch-22 of having Heart Strike be the primary AoE rune ability in our kit. Additionally, since Heart Strike generates more Runic Power, using Heart Strike during periods of high incoming damage allows you to use more Death Strikes, which has synergy with the high incoming damage because its heal scales off damage taken.You shouldn't worry about using offensive abilities (like Heart Strike) during periods of high incoming damage.
Yes, this would reduce the effectiveness of Heartbreaker per use of Heart Strike, but that nerf is counter-acted by the fact that you would also be able to use more Heart Strikes than before (as you would be committing fewer runes to Marrowrend usage), thus netting no significant change to Heartbreaker's effectiveness. As an added bonus, however, this change would also make talents like Spectral Deflection and Tombstone more viable since you would have an increased rate of Bone Shield generation (with poor Bone Shield generation being the main drawback to those two talents).This change would be an unnecessary nerf to Heartbreaker (Talent 1,2) because you would be losing 2 potential targets to gain bonus RP from.
Well lucky for you I have a suggestion that accomplishes something similar :P