i like irrlicht but currently none of the supported 3d-formats suits my needs so i would like to add my own.
i thought of something like this:
the world format is an ascii file with information about all level polygons. two uv sets get used so that i can have shadow maps. textures are kept external in .tga or .jpg files. the file also contains the position of static entities like trees.
the model format basically should be an ascii version of quake mdl but without its precision limitations. later i would like to add more features to it.
...
how hard is it to do something like this? (i am more an artist than programmer and not very experienced with c++ but i am learning.)
adding support for new 3d-formats
A bit ambitious to design your own model format then, eh 
Well, I think it would be hard but possible. If I was you tho m8, I'd just use one of the existing model formats. I know a bit of C++ and I wouldn't bother attempting to design my own model format, if you aren't really confident in C++ ( or even if you were ) I wouldn't bother. I'd use the existing model formats, but create my own map file for expressing where entities in the world would go, such as tree's and avatars, stuff like that. XML is a good candidate for that, it's what I am using and seems to keep the files quite easy to edit.
Well, I think it would be hard but possible. If I was you tho m8, I'd just use one of the existing model formats. I know a bit of C++ and I wouldn't bother attempting to design my own model format, if you aren't really confident in C++ ( or even if you were ) I wouldn't bother. I'd use the existing model formats, but create my own map file for expressing where entities in the world would go, such as tree's and avatars, stuff like that. XML is a good candidate for that, it's what I am using and seems to keep the files quite easy to edit.