to have 2 5d3's so I will have a backup.
Do you do mission critical shootings all the time like weddings where you need a 1:1 instant backup, not once in a while where you could rent or borrow another eos camera? This type of gear isn't very prone to failure anyway, you know :-p
My main want is stop better iso performance, customizable ae adjustments, more accurate metering.
Sure, seeing you enjoy juggling with gear and have the cashflow to back your preference, just get the 1dx!
You then will have the absolute top of the line gear which surely will be fun, I'd like to play around with it too. My point is: This will help you to realize what gear can do and cannot do, i.e. what you need to learn for yourself (unless you wait for the 1dx2 :-)) and might make you enjoy photography even more, even if someone shout ever put a 550d + 50/1.8 in your hand.
So far events and portraits are all I have done. Also landscapes and macro.
So what *didn't* you do :-> ?