This is the right direction to take, and offers more flexibility than Spellmaking ever could, it just simply wasn't utilized to it's full potential.
What makes you think spell creation was utilized to its full potential? It suffered somewhat from only having three types of spell delivery, but with the spell delivery methods in Skyrim, a spell creation system would've provided mage players with a ton of freedom. How can less choice be the right direction in a sandbox game?
You mentioned how spell creation wouldn't have provided more mage builds. Well, what about the streamer-mage, who only uses flamethrower-type spells? Spell creation would've allowed such a mage to not only use all the elements but also remain viable.
What about damage-over-time combat mages? Cast a relatively cheaper DoT spell and then tank it out as the enemy runs out of stamina or health. Cast a magicka-drainer on the wizard and then tank out his damage spells behind your own magic and elemental resistance.
How about the trap-based spellsword or spell-rogue who could have all sorts of fun sneaking about and laying down various kinds of runes as either distractions or opening moves? Not much that patrolling wizard can do to avoid a backstab if he walks onto a paralysis rune and if he resists then you haven't committed to open combat yet and can try something else.
How about an archer that throws out a zero damage "ball of slow" to make hitting his targets easier? Or places a weakness to fire rune before revealing his position to the bunch of draugr that he's planning to introduce to his fire enchanted bow? And don't tell me to use poisons instead, because that's an alchemist trade-mark and it's possible that someone would like to RP something that isn't based exclusively on alchemy. Using spells is way cooler than using potions, after all. And weighs a hell of a lot less, and is with you all the time when you've got magicka