There is a saying in Cyberpunk of William Gibson’s, that the street finds its own uses for things. The idea is that people who make their living on the streets, the criminal underbelly, will come up with their own uses for the various technology that gets into their possession. The main quote example shows below to see the sort of idea in use in cyberpunk.
If your main squeeze has just decided to walk out on you, booze and Vasopressin are the ultimate in masochistic pharmacology; the juice makes you maudlin and the Vasopressin makes you remember, I mean really remember. Clinically they use the stuff to counter senile amnesia, but the street finds its own uses for things. So I’d bought myself an ultraintense replay of a bad affair; trouble is, you get the bad with the good. Go gunning for transports of animal ecstasy and you get what you said, too, and what she said to that, how she walked away and never looked back.
You can see this even in today’s world with the DIY hack-a-day projects that people come up with to do things. Johnny Wu had shown how to take a Wii Remote and receiver and do motion tracking, for example. An in-genre example, Strange Days where the SQUID was basically an electrode net worn on the head which recorded what you saw and heard and felt to a data drive. It was then co-opted by the criminals to allow for recording of various clips and sale of these moments of life, so you could pay to BE the person doing things, like some of the examples they show in the movie are of a guy as part of a crew commiting robbery and a woman in the middle of having sex.
This is something a crafty GM can get a lot of milage with. In rulebooks you can find some things that may seem like why would anyone want this so your players never take it. You can show how it could be useful in a different way than obvious. Shadowrun had a magnetic hand feature, and someone later took the in game cyberskates and added rules for skitching like in Snow Crash as a fan addon, even making magnetic harpoon guns Y.T. uses. It didn’t directly need a house rule but it added a new genre to the game, and the harpoon allowed for some Just Cause style situations especially when coupled with in game rules for parachutes.
You can have NPCs doing cool things the players never expected, but they have to flow logically from established rulesets. For example, in your system it increases duration to cast a spell again on a target, don’t suddenly have someone able to add additional multiplers to another value. It can unbalance the system, especially if your players cannot use those same rules to benefit themselves. If that’s the case, it smells of GM fiat, as you are only letting your NPCs do certain things that should be available for everyone.
I think what works best is figuring out ways to make the general idea the same, but have it being a little different in usages. For example, if you watch 6th Day there is a scene where one PC has a holographic chair which creates a virtual girlfriend and he mentions to the main character ‘If your senses tell you you have a hot girl in yout lap then you have a hot girl in your lap.’ Convert this to a magic environment, you could use spells to create an illusion of a girl and either have it directly affect sensations or add some other spells to do it. Makes for a great brothel in a high magic area like Sharn in Eberron. They pay for the spells once and don’t need to pay again, and the girls can be tailored to the client.
Another slew of examples come from the Knights of the Dinner Table, such as the fight between Tremble and Carvin Marvin, but as that was a large story arc with multiple such examples, I will use the simpler example with pitbulls that became The Doomsday Pack. Previously, the players found what was likely a typo stating there was 8d100 hunting dogs in every town. They went out and bought a few hundred for combat purposes and the DM had this backfire later when, after all but one PC was captured they mounted an agressive rescue with a large pack of hunting dogs against the guards of the prison caravan. After the dogs were chased off, they began growing at each town they would wipe out, slowly destroying the whole world. It’s a little grim, but the idea being using the rules as written allowed the players and then the GM to come up with solutions to problems they were faced with.