longer cables & layout - see above... you may find that you want to use your current 6u 104hp case as a control skiff in front if the 15u is vertical - I find that I prefer sequencers, controllers, end of chain mixer etc are best horizontal and at the front

OK, thats probably ergonomically better to have it horizontally or what is the matter?
The Case is not entirely straight. Its straight at the top but concave towards the bottom.

I didn´t think about new cables that will add cost of course.

not really, more an extrapolation of what I was doing with a smaller case... more voices to patch (potentially) - use more utilities to make the most of modulation (multing, combining, inverting, attenuating etc) so less modulation sources can be used to modulate more modules... with related modulation... so things fit together better...

Sounds like the way I like to work anyway... nice

Do you work on patches for a longer time than in a small case?

probably - they take longer to patch - and take longer to finesse

Would you say they got better though?