Rather than alter Bane, one of the new Egi could be given an ability that has the same effect and CD as Bane. If you had to choose between Contagion and the new Egi-Bane perhaps the devs wouldn't consider that being overpowered.

What gets me though is after playing a decent geared BLM and AoEing the crap out of massive hoards I can't imagine why SE thinks Bane spreading to everything would be so much worse.