To lend more weight to the concept of paradigms, make the range of possible effects achievable with a sphere independent from the mage’s rank in that sphere, while simultaneously coupling the rank to the restrictions imposed on the use of the sphere by the mage’s paradigm:
Even with a sphere at • a mage can already do “everything” (associated with that sphere, of course), but only by slavishly adhering to all the ritual shackles imposed by their particular paradigm. With a sphere at ••••• they cannot do any more than before, but only now can they finally do it by simply willing it and nothing more.
(This also should encourage you to work out what those paradigm shackles actually are and think about how your character learns to transcend them one by one. A possible rule of thumb is to have four different limitations for each sphere and also to have them not only add flavour but also impose actual limitations, such as on time required, ressources consumed, or just plain old standing out like a sore vulgar thumb.)
Little Rules are these tiny bits and pieces that just seem to creep up naturally as a game goes on and players dive in. Not the big changes or additions, the little ones.
(Although I admit that this time this could also be seen as a pretty big change right here.)