Nope. Make no sense in mmo.
Spellcrafting was balanced in Oblivion just fine...you either had a spell that would do good damage and you could cast once...or you had something more utilitarian. The big difference between Oblivion and ESO is that there was no magicka regen in Oblivion...you had to rest or drink a potion to get it back. I honestly don't think balancing it would be any more difficult than other skills. I do think that it might actually promote hybrid builds though, at least for PvP where someone will open with a powerful magicka-heavy spell, then be forced to swap to stamina spells to continue the fight. My prediction is that in PvP initial burst damage would be higher, followed by weaker abilities, which would force people to change the way they play, with possibly damage reduction spells that last x amount of time cast between fights to prevent the initial burst...running into a fight without proper preparation would be more dangerous, but properly done they could make effective counters for every conceivable combination. Gankers increasing burst damage? The counter would to to have an AoE stun field around you that would trigger when a stealthed opponent came in range. Maybe they would craft some jesus-beam like spell for non-templars...a possible counter spell could be some type of channeled ability reflect that would reflect it back at the caster. The idea is that the possibilities are endless and that anyone in PvP would need to have an extensive library of spells and counters, which they would tailor to the fight at hand...they die to a particular setup...change skills, then easily overcome that setup...causing the defeated opponent to reconsider and change skills. The fights would be far more interesting with anyone able to change their toolkit dramatically from one fight to the next. In practice most people would run the same half dozen spells and their counters because Meta, but there would be much more flexibility to deviate from that path. Also...skill-crafting would also provide counters to existing abilities people complain about constantly like Zaan, or such and such a class having no mobility, or too squishy, etc.
Wish there was an 'other' option. I kind of would like to see a proposal on adding it, but with the difficulty in balancing the game the idea scares me that it will add more problems. But if done right it would be cool.
I am not a PvP player and I don't think they should bring it in at this point.
I could see it coming into play, back when the game first began and players were actually balancing their main stats between magic and stamina. However, now that the game pretty much depends on a player choosing one OR the other and not both this would be debilitating.
(Insert but..but.. but the spells could be made to reinforce your stamina blah blah blah blah.)
That's not what spell crafting is about. It is about a second skill line based entirely on magic. And I would only support that if they had an additional skill line based on stamina (2nd fighters guild since this is based on mages guild) that came out at the exact same time. Better yet just replace the original two entirely with the new ones.
I have been through at least one other game (not Elder Scrolls related) that had a very in-depth spell crafting mechanic. It ended up being a lot like alchemy is in ESO in the aspect of you used different materials to make different types of spells and could only learn them by mixing them together.
Now I know that's not what was suggested above however it wasn't more than three months before people figured out exactly which spells were the meta to craft. Let's face it no matter how ingenious or how creative you might be for the spell to actually be created it has to be programmed in. That means you aren't really creating anything; it means the possibilities are finite, it also means that one or two spells will be selected out as meta and that's it.
So how many months or years of balancing for three months of content that is only going to get two maybe three spells used?
Sorry but while I absolutely love the idea of new ways to "customize" my character through a plethora of skill lines, ZOS has proven time and time again that they just don't do a good job of keeping all these skills balanced in a way that truly supports that customization.
There are currently way more skills available to help gimp your toon to an unplayable point than skills that will actually help make it playable. I really wish they would spend time bringing the broken skills we have up to par than creating new ones.
If this was implemented in the original game... perhaps it might have worked but now... not so much.
Personally...
Rather have the rest of the Justice System (that was ripped out) put into the game!
Wish there was an 'other' option. I kind of would like to see a proposal on adding it, but with the difficulty in balancing the game the idea scares me that it will add more problems. But if done right it would be cool.