Out of curiosity, what are you finding lacking in blender's 2D capabilities? Usually when I find blender missing something I need, I just haven't looked hard enough :P

I get from the beginning post that it seems you are interested in something comparable to Blender, but 2d. It seems like you want a vector program. Illustrator is king in most circles(though it has a price tag) while Inkscape is one of the best free options.

Other 2d art programs are raster programs, which are based on pixels instead of Vectors. There are many more of those, but I don't go into details because it isn't what I would say is "Blender alternative, but better 2d." Honestly though, I'd think Blender can handle 2d just fine too. You would want to learn the hotkeys though, for things like constraining movement on certain axes.

Misantes, i miss filters, layers, blend mode and brushes
Kburkhart, you didn't got it

Well, I don't think what you want exists. Software is usually specialized at something, and then lots of it has something else that it doesn't do as well. Blender is pretty good all around for 3d modelling, but though it has sculpting and texture paint, it isn't that great at those, at least as compared to other programs that do it better. Photoshop is excellent for 2d raster art, but though it has a few vector tools, it isn't that good at it, which is why Illustrator exists.

I think it is better this way, than having programs try to just be the be all end all kind of things. It is hard to be really good at more than one thing. Instead it is good to be the best(or top side at least) of one thing, maybe do a little of others, and let the others be best at other things. Then, we can get all we need, though we might have to use more than one program. And we can get really good quality this way. Blender is great at modelling, and I use it for that. I sometimes use it for texture painting too, but I'm liking this trial of Substance Designer and Painter and will likely be getting those soon, as they are really good for texturing things, much better than Blender for the purpose.

Misantes, i miss filters, layers, blend mode and brushes
Kburkhart, you didn't got it

Depending on what exactly you mean, blender does have layers (plus you can create vertex groups to mask anything you need), blend modes, and while the default brush set is pretty basic, you can import your own or import textures to use (as either textures or a texture-mask). Depending on what you mean by filter as well, blender likely has you covered too. The documentation is super thorough and includes tutorials for most things. Additionally, look in the preferences under plugins, as there are a ton of features turned off for simplicity. Some plugins aren't included, but are incredibly handy (like importing brush sets, is a must-have for me), but they're rather easy to install.

That said, it certainly doesn't have everything you'd need. Personally, I use blender in tangent with Gimp and/or Krita to create textures, exporting from one to the other. To my knowledge, there isn't a single 2D/3D program that contains all of the functionality of blender/maya with the functionality of gimp/photoshop. If there is, it probably isn't cheap Like kburkhart84 mentioned, it's not a bad thing to use multiple programs for some of this stuff. Use the tool that's suited for the task :)

I'm not here to sell blender though, and there are certainly other programs that have all the same functionality. I'm just suggesting that perhaps the features you're looking for are already there. Again, usually when I think I'm missing something in blender ,it's usually there and I just haven't found out how to access it. The same goes for Maya, 3D-coat, etc. They're all rather robust, feature-wise, but along with that comes some complexity and it's not always intuitive to do what you want to do. Generally, I just google it, and there's almost always a solution.