I would like a guideline for coming up with guidelines 
Specifically for conjunctive effects.
I always stick to generalists casters and spontaneous magic because I'm largely uncreative when it comes to making a formulaic spell that is useful enough to make it a formulaic spell. Then after I play I come up with a great idea for a spell, but my characters kinda suck and won't be able to invent it because the guidelines say its a large magnitude spell and I almost never specialize.
The guidelines listed in 5th ed are much better and more in depth than the 4th ed, which is great. But I would like to see more. My particular pet complaint has to do with Ice. Perdo Ignem has a spell to freeze someone solid, with no corpus req... but other forms more suitable for freezing in ice are higher magnitude, freezing someone solid would be something Corpus with Terrem and Aquam reqs or any combination of them which amounts to the same. You need 3 high Forms to invent and cast because they don't work together to create a conjunctive effect.
While my example is Ice specifically, there are other effects that are a combination of Forms that I would like to see be easier because they combine them.
Combining Arts, more specifically Forms increases the level of a spell more than the increase of effect gained. They don't work together to create better effects or make an effect easer to accomplish. This also inadvertently pushes the player to rely on spells from their specialty and thus be dependent on their 'spell list'. For instance if you are creating a Creo Aurum spell to make a localized storm, a general unnatural storm is magnitude x, if you add an Aquam requisite to make the storm a cold sleet storm that covers everything in ice. The spell is higher level than the more general "Make a storm" spell and then specifying sleet. And the specific Sleet Storm spell is harder to cast. Likewise, if you went the other way and did Creo Aquam to cover everything in ice and then added an Aurum req. for some cold wind. Yes the spell with a req. is going to be more potent, but it’s pretty specific.
When coming up with a character concept you ask yourself, what do I want to do. More common than not the thing you want to do most is based on one or two Art combinations at a high level. So you specialize to do the one or two things in your shtick and then you weigh the 'spells in your list' and merely progress learning spells in that List. Ie Creo Ignem or Perdo Corpus. Sometimes you grab a useful utility spell based on one Art you are strong in. The character rarely picks up spells with requisites because as a specialist they have very few other Arts they are accomplished with.
One good spell mastered is better than any amount of creative spontaneous casting. Because of that I see a lot of stacking arts to do the one thing well, then pick spells from the list and master them and that’s all you do. In that case the game unintentionally becomes 'spell list based' because it encourages it. By spell list I'm talking about RoleMaster type spell lists.
Part of the problem in creating spells is the progression of magnitudes, after a spell breaks lvl 5 additional range etc increases by 5 levels. So any spell based on an effect lvl 5 or above becomes really high level, requiring a specialist. This encourages specialization to be able to do 1 cool thing, but then everything else you do is from the spell list of that Art combination. (But if you are going to be a specialist, just do Ignem, Perdo, Creo, or Corpus and leave the others alone, 
)
Some rule to lessen the level of spells with conjunctive effects is needed to make the spell system more like it is intended. Right now I think it encourages over specialization and just picking from your list. IE like Role Master where Creo Ignem is Fire Law.
