well it would be like buying a spell then improving upon it slightly, or making it a different function.
buy fireball and make a stronger flames for example.
So... it would be completely pointless.
I'm not saying Skyrim's spell system is perfect; while it's certainly good, there's plenty of room for improvement. Spellmaking, however, would not be an improvement. I feel that they should continue the route they took in making each spell effect unique and useful, but they need to make Destruction spells scale in damage (why does my character do 402 damage per hit with Chillrend with fast swings at 100 One-Handed, but when I switch to Firestorm at 100 Destruction I only do ~200 damage, with a 5-second casting delay between uses, and a high magicka cost?), and there need to be more varieties (Command Humanoid/Creature, Absorb Skill, Bound Armor/Dagger/Shield/Mace/etc., Chameleon, Damage Attribute, Cure Disease/Poison/Paralysis, Silence, Dispel, Feather, Fortify Health/Stamina, Unlock, various Summons, Slow Fall, Swift Swim, Water Walking, and a few others) could all work in Skyrim's gameplay. I think the solution would be to give multiple casting options per spell, e.g. Conjure Bound Object could be used to summon any piece of conjured armor or weapon based on the mode selected. Having all the spells grouped in one spell makes for less spellbook clutter in the menus and more incentive to buy and use a spell for various applications.
@EffeminaT: You miss the point. It's not true Spellmaking if you just buy an effect and use Spellmaking to make a numerically stronger but otherwise identical spell is pointless and spreadsheety. The problem with having a system that allows the player to make new spell effects is that the developers would have to basically design the spell and put it in the game already, but make the player unlock it through Spell"making".
EDIT: To clarify, I'm basically saying that fixing the non-scaling Destruction damage by adding a system that lets you scale up the numbers yourself is just fixing a problem with another problem.
I do believe that TES should have some system that allows you to make spells, but the previous games' "scale up the numbers of a spell and use a stronger-but-otherwise-identical-spell" is no better than just having Skyrim's system but with spells that scale in damage with levels. If someone could contribute ideas as to how a Spellmaking system for creating new,
unique effects could be done, please, feel free to suggest some ideas. Seeing how an entire professional dev team drew a blank on the subject, I don't think it's going to be possible.