Yikes, I'm cool with I4's, but the V6 thing just isn't something I'd be interested in. I think that's all rumor at the moment for the M3 and will likely not spread to the non-M line in any manner. At least I hope not.
I'm not so sure I'd say that the N20 is the equal of the N55 as it gives up 25% in hp and about 20% in torque. I'd imagine the N55 will stick around as is for the first two years of F30 production and then will be upgraded/replaced. The N20 is impressive, but not something that's likely to replace the N55 directly.