You should be able to breakaway whenever you have 2 def bars.
If you happen to get hit by a KB, like a d2 KB, Jade's forward throw, Erron's 21212 kounter hit, etc., I believe you should lose the ability to breakaway whatever comes after.
Consequences:
-dents the strike/throw meta by making throws even riskier
-matches would be overall faster
-wake ups and cancels (Mean Streak, Sindel, etc.) would more often be available
It would greatly benefit characters like Shao, Jade, Kotal, and Scorpion too, whose KBs often serve as combo starters themselves. Compare to other characters such a Geras whose KBs are unbreakable kounter hit strings. Or Noob, whose tackle KB occurs at the end of a combo.