Great feature in Oblivion that was removed for Skyrim. As long as you knew the spell, you could craft your own. For instance, I learned Weakness To Fire, and Fireball. I may have the name wrong on Fireball (it's been a while). I crafted my own spell with both. So I shot a Fierball that gave them a 10% weakness to fire, and burned them for 5 seconds. It may not sound like much, but I pretty much shot it at the mob, and watched it burn up as it ran to me, only to drop dead along the way!
Those spells are my favorite ones!
There were many fun combinations. Like, Paralyze-Fire Damage, Levitation-Burden, Fortify Acrobatics-Fortify Athletics-Fortify Speed-Fortify Agility

I want it back. I do not really see the problem, they can allow you to modify:
How it's cast (stream, bolt, rune etc.)
Effect (Fire damage, paralyze, fear etc.)
Area Of Effect (0ft, 10ft, 20ft, etc.)
Magnitude (10p, 20p, 30p etc.)
Duration (0s, 5s, 10s etc.)
Not really complicated it seems.
And for the Dual Cast issue, you can't combine diferent spells anyway, so if you want to cast a spell with Fear-Frost Damage you simply cast it in either one hand or two hands. Trying to combine the visual effects of those spells seems easy enough.
They can make it such that you can't Dual Cast if it have multiple effects.
(Like voice of reason, I just knew Merari would come by

)
Although, I have looked into spells in this game, and they are all scripted events it seems. That could maybe make complications when I think of it, but I believe Bethesda could manage to fidn a workaround.
A DLC that allowed spellmaking would be bought in an instant. Easely 10 bucks.