Nice idea (which I had myself, too :D) but I guess that’s one step too far at the moment and requires much more effort than these basic changes I make at the moment. First I’d like to know that my work is accepted by the Radiant dev team before I’ll probably go on with more options like grouping textures which will never be available for the community 
I dont mean have you do it all, that would be harsh, but allow the user to sort it, and maybe save it to a external file? (so people can post them here for lazy people (like me! :P)
That’s right. I guess I would be too lazy myself to sort around 500-600 textures, instead I’d try to build a XML based solution so that others could sort it out 
or maybe a search function?
so you could search for ‘stone’ and get all textures with stone in name! 
I’ll put that on my wish-list. There’s a basic search function already if you have “texture subsets” enabled which works for the current texture set but not for all available textures.
I’m not sure about this idea because
a) it might need too much memory to display hundrets of textures (even if they’re shrinked to thumbnails)
b) I think lots of textures would lose too much detail when they’re shrunk to be identified
Also might be useful for a user ‘palette’ with a selection of textures that the user can add to the palette whatever texture he or she wants, so that frequently used textures are more easily accesible.
Hmmm. If you work on a map and have textures or shaders in use, you see them all by hitting “hide unused”. That’s almost what you suggested. I think your palette idea complies (more or less) the grouping idea 
Are you thinking about hiding custom pk3 files? I only worked on Q3 based games with Radiant so far where it’s suggested to use a clean installation for mapping. I think it would be definitely the wrong way to bypass this rule because it might cause other errors if someone has custom pk3 files in his mapping folder 
Same here. I still use 1.4 because some important things are still missing. SPoG has been working on the bobtoolz for example and I assume they will be available with the next release. I think this could be the point where I start using 1.5 
Will this work with the older version?
No. AFAIK versions < 1.5 won’t be changed anymore. The source code for 1.5 is available which allows everyone to make changes and additions.