iMac wasn't an option, and likely for good reason ... for example the OP may not want to be tied to his desk by any computer ... makes sense to me.
I'd suggest two options
a) your 11" MBA and a new 15" MBP, (17" too big for anywhere but a table/desk)
or
b) by a new 13" MBA and use it for all your work, easier than juggling two machines. You could use external HD's for back up, or if you needed an always on computer at home get a mac mini or the iMac's suggestions above would work.
I love my 11" MBA probably my favorite notebook so far, but I admit I'm far more productive on my 13" MBA. The 13" is not quite as amazing to use on my lap or arm of a couch, table, car, cafe or desk as my 11" is. It's still very very portable and for many the max'd out 13" MBA could likely serve as a full time computer.