Expedition: general discussions.

I understand the main idea is that you can create your own spells and use them.

Haven't you played Morrowind? You can definitely create your own spells there. The main problem is that most of the magic functionality is overlapped by enchantments, which serve as the same customizable magic, but are built into items (you need to equip them and they consume enchantment charges instead of mana). Moreover, enchantments, unlike classic magic, work in any situation and have no chance of miscasting. Healing can also be easily compensated for by alchemical potions of your own making, which also have no chance of miscasting and are much more effective in themselves. You can also add an enchantment for permanent health regeneration to this.

Added 1 minute ago
And the main overpowered build in Morrowind isn't a mage, by the way, but an enchanter with a permanent 100% chameleon effect.

Added 4 minutes ago

Yes, enchanting isn't magic at all from a gameplay perspective; I already wrote about it. The problem is with castable magic (although at early levels, it's probably useful to use summoned weapons, as they have stats comparable to Daedric weapons, and various atronachs, but this is one usable school of magic among many that are not very useful).

While I agree that warriors scale better, thanks to the base weapon damage from smithing and damage buffs from enchanting, I wouldn't call mages weak. I easily completed the game as a pure mage, who actively used only destruction magic, and didn't notice any problems in battles with enemies – various Alduins went down in a couple of seconds. The only problem might be elemental resistance, if you decide to specialize in only one element. But even so, using mostly electricity, I had to switch to another type of damage only once – during the battle with Morokei.