Before looking at the next camera, make sure you have the best lenses for your needs. A good macro lens for example will make much more difference than a change in camera. The same is true of long fast lenses.
If you have enough good lenses though, it gets hard to change camera brands, so the first factor to consider is if you are willing to change brands should there be something more suitable by another manufacturer. Sticking with the same brand, makes the choice easier :)
Higher-end models are primarily made to let you work faster. You mentioned sports and wildlife which are excellent use cases for this and this says that you should consider faster frame-rates as of higher importance than higher-resolution. Canon knows this too and has made their faster model (EOS 1D X) much pricier than the highest resolution one (EOS 5D Mark III). Nikon did the same with the D4 (fastest) and D800E (highest resolution).
Portraiture and macro are less demanding but if you do extreme macro, Canon is the way to go, they have the only DSLR-compatible 5X magnification lens, the MP-E 65mm F/2.8. Getting the same magnification with any other brand is quite inconvenient.
For studio portraits, depending on your lighting equipment you can look for something with a sync-port. You will notice a Sync-Port appears on the EOS 7D and higher end models in the Canon lineup. The more modern way of shooting with studio lighting is to use wireless flash triggers which work using a standard hot-shoe rather than sync-port.
The 7D is also well suited for action with a frame rate of 8 FPS. Not only that, as a cropped-sensor camera, your lenses get more reach which is a great bonus for wildlife and certain sports.
The closest equivalent from other brands are the Nikon D7000 and Pentax K-5. Nikon, just like Canon, lets you grow further from there with full-frame models higher in the lineup. They also have an extensive line-up of lenses where Pentax much fewer and is missing long telephoto lenses which gets them complaints from wildlife photographers. Sigma fills the gap somewhat.