Why? If you shoot out the lights with a gun that could be a maneuver placing the dark aspect on the scene. If you throw a molotov cocktail into the middle of your opponents it would place the on fire aspect there. I would say the only difference between evocation maneuvers that affect scene aspects and regular ones is that the evocation maneuver has a defined duration.
The difference here is that the light bulb was a pre existing condition of the scene that had to be there to make it happen in the first place. Magic is creating something out of literally nothing. Saying that a grease spell immediately effects the entire scene puts it on all zones at the same time and makes it usable against the caster. The intent isn't to cover the entire football field with grease, only the 50 yard line. Besides this I don't recall any rules for having magic effect the entirety of a scene, only for zones. Consequently there aren't any rules for having magic maneuver on a zone wide level either. But magic can maneuver on a single target and magic can attack on a zone wide level, so it stands to reason you would be able to maneuver against multiples.
This is the assumption I don't understand. You have created a single aspect. Therefore you get one single free tag, not one per affected person, to use or pass. Yes, the next time they act they will have to contend with the new scene aspect, and may be compelled by it, but I think it would be a regular compel which they would get a fate point for accepting.
Your scene aspect logic is what confuses you in this respect. You're not allowing yourself to see it in another way. As in aspect on the character just like normal maneuvering works. When I maneuver behind someone to flank them it isn't a scene aspect! It is attached directly to the person, the same goes for single target magic maneuvers. Multi target maneuvers should do it in the same fashion rather than throwing it on the whole scene. The cost DOES need to be appropriate though.
I thought carefully about making it a zonewide block as well, but that didn't make sense either once someone breaks the block it is gone, so once one person makes it across the slick now everyone can? It immediately dispels? =/
I think I have a fairer idea though to make the cost match the effect. This is for evocation mind you.
1. The caster has to determine the amount of targets. If rote this will be pre determined and set in stone as to how many it can effect at once.
2. The spell Will cost -2 per zone + -3 for maneuver effects + -1 For each target.
( So an AOE 1 target would be ludicrous an ill advised. A 2 target would be on par, and savings at 3 target)
3. Duration would be added at -1 per target + -2 per zone.
4. Just like a block, power could be fed in each turn. A rote spell could only be fed into if another rote spell existed for that sole purpose.
5. Each target gains the desired aspect that is a free tag once, only casting again would allow for another free tag.
6. The target gains a fate point each time the aspect is used against them.
7. The aspect may be used as either a compel at cast time or tag for +2 attacking-2 defending or re-roll if attacking.
8. Additional targets walking in will increase required power automatically, this can easily throw you out of rote spell mode. If you can't control the excess power and go into backlash the spell continues, fallout discontinues the spell immediately and does not effect the current targets.
I think this limits the scope of the concept enough to make it useful but not so powerful as to effect 500 enemies in the same zone for 5 shifts of power.