All of the above - depending on the piece, the purpose etc.
If the piece is to showcase the pianist, then the composition will be technically difficult. Actually to make ME (the composer) look good, I would actually tailor the piece to bring out the strength of a particular pianist depending on that pianist's style - and of course get that pianist to play it. If I write it for myself, I know my stengths and limitations and therefore the piece will make ME (the player this time) look good.
If the piano part is an accompaniment, then it will serve to complement and not to confuse the piece. Again, it will depend on the piece.
I did a musical with some piano parts. The main focus is the singing, so all the music is to complement the singers. The piano part is therefore arranged to complement the rest of the orchestra as a whole. But the orchestration is still quite thick with drum kits, e bass etc - but never to drown the singers.
I also did a number of pieces for wedding processionals. The piano parts are in these cases, simple and usually as the main instrument with the rest of the orchestra serving to bring out the melody.
The key is still theory and harmony. Complicated pieces may not make it sound "rich" - it may make it sound messy. My piano arrangements are usually in 3 parts, very much like Bach's 3-part invention concept. That means, the piano will actually play 3 different simultaneous voices/parts at the same time. If it is a solo piece with no accompaniment from other instruments, the piano will start to emulate different instruments. Tympani rolls, fleeting strings, harp, even brass can be "emulated". The piano is indeed a most amazing instrument - the possibilites are endless.