I'm pretty sure I have seen a Virtue about Spontaneous magic that allows the caster to "build up" power over subsequent rounds, expending fatigue each round. Something like ... if you build up power for two rounds, your casting total is 3/4 your casting score, and if you build up power for three rounds, it's equal to your casting score. But I can no longer find it, and I am starting to suspect it might be from a previous edition.
Can you help me locate it?
Just found it. It's Continuous Spontaneous Magic, a +4 Virtue from the 3rd edition corebook. Hmm. I wonder if it would make a fair Major Virtue in 5th edition...
I think it might make a good Major Breakthrough, as it feels fairly close to Diedne Magic in power level. It allows a potentially higher spont at the cost of several levels of fatigue/turns of casting.
vs
It actually feels like a much more brute force method of getting more powerful spont effects, compared to the Diedne method which feels more effortless. With this Virtue you are pumping a massive amount of energy into the spell to get the desired result. Diedne Virtue meanwhile feels almost as if the Arts are Abilities, into which you can put a little more effort to get a powerful effect.
Also you might be confused with the Diedne because your Spontaneous Magic is so powerful. Could lead to some interesting stories.
Think very carefully before allowing it. In 3rd and 4th edition this was a very exploitable virtue (particularly combined with Life linked spontaneous magic). The most powerfully abusive characters I knew of used this virtue to be able to do almost anything.
Although charging a fatigue level per round of build up would help it still does not prevent a mage doing very impressive things
If you decide to go through magical research to reach this virtue, there is a faerie method that does something very similar in Realms of Power: Faerie. That would probably be a good place to do some research.