My mk6 GTD was a great car too; one of the best cars I've owned.
I went the opposite way to you and downsized last year and bought a Polo GTI. It does all I need from a car as I rarely carry more than 1 passenger. It's pretty quick, fun to drive, fits into small parking spaces and it's quite frugal if you're sensible!
If I needed a large car, then I think the A5 would be on my list of possible replacements.