You'd make something that people would either want or that what you feel is an improvement to the game, although im pretty sure all the better mods try to add something new or expand off of something to make it more fun
Like mekanism, minecraft hadnt seen a sort of clockwork-like mod like that before and everyone really liked it because it fit minecraft's survival/no advanced technology feel.
having to resort to clockwork to make things better has a very good feel for a survival game because its easily human made not by machine
thats why we liked mekanism, because it fit and introduced something people wanted and needed
