Aaah, Puella Magica. I never really watched it myself, but I am somewhat familiar with it and at the very least have some experience with these kind of "gimmicks" from various canons. The answer I'm about to give is a bit general so it can be applied to other canons that might be in similar situations, and then I'll further specify how it would work for PMMM.
Time Stoppage. First, the time ability to stop time. As you said, traveling back and forth through time is completely out (but who knows, I could always pocket it as a potential plot event—it would actually work very well for this premise in that case). Temporarily stopping time can be done. It would be for a short duration (let's say 5 minutes max) and for a limited area (let's say the immediate area, maybe 50 feet). It would also have a bit of a "cooldown" period, meaning the character wouldn't be able to constantly chain time freezes. It might also take a bit more energy to perform than usual. That isn't to say it would make them super tired, but more energy loss than they would be used to. Again, these limiters can change depending on events and the progression of the game.
Some other limits to consider are those that are affected, and this is really going more toward player permission policies. From the NPC side, I have no issues of NPCs being affected by time stopping. But for PCs, ideally, you'd want a situation where it's those who are supposed to be directly affected and not have any collateral victims. If you do want to do something that's large scale, it would be allowed, but an announcement to the playerbase would be needed as well as an opt-in. This could mean that not everyone in an affected area is actually frozen in time.
Hope that makes sense! If not, feel free to ask and I can try to clarify.
Soul Gems. There are many different ways to go about these kind of items. In general, I let the players the decide how they'd like it. The options are the following:
• The item exists and just works in an optimal way. It all works or exists, but there's no need to worry about keeping a balance. An example would be vampires—in this option, they wouldn't have urges to feed on blood at all even though they still could. In your case, the soul gem would just stay clean all the time, so it would be like the "stasis" method you mentioned.
• The item exists in its full capacity, but an alternative is available. This would be what you mentioned as "world provide something else." In general, it would be a kind of direct alternative. Back to the vampires, they would just eat normal food. For metaphysical needs, such as mana, emotions would be the alternative. In your case, the use of the emotions might be the best alternative. To put it more in line with the canon, what you could do is have the character find other negative emotions and try to purify it with their own positive emotions. These negative emotions would basically replace the witches.
• The item exists, but is completely neutralized. This is a little similar to the first option, but also very different. In this case, you're effectively sealing the ability completely, so it's like a complete nerf. This should really only be used with things that, while important to the character in some respects, isn't actually necessary and its "disappearance" would just allow the character a better chance at living a normal life. This can work in your case, but I'd advise against it as it does risk altering how the characters work greatly. It would be different if the soul gem was just some external source of power and that's it, but as I understand it soul gems also help define them in a way.
So those are a couple options you could do, and you could mix it up a bit, too. One thing I'd note is that you could keep the aspect of the soul gem getting tarnished. You could this in one of three ways: 1) tarnished by the character's own negative feelings, 2) tarnished by the negative feelings of others (strong ones, of course), or 3) a mix of both. This way, you can keep a kind of dynamic if you opt to have the soul gem still working in some capacity and open up some more potential plots, be it personal or gamewide.
Phew, sorry for the long answer! But hopefully that helps.
no subject
Time Stoppage.
First, the time ability to stop time. As you said, traveling back and forth through time is completely out (but who knows, I could always pocket it as a potential plot event—it would actually work very well for this premise in that case). Temporarily stopping time can be done. It would be for a short duration (let's say 5 minutes max) and for a limited area (let's say the immediate area, maybe 50 feet). It would also have a bit of a "cooldown" period, meaning the character wouldn't be able to constantly chain time freezes. It might also take a bit more energy to perform than usual. That isn't to say it would make them super tired, but more energy loss than they would be used to. Again, these limiters can change depending on events and the progression of the game.
Some other limits to consider are those that are affected, and this is really going more toward player permission policies. From the NPC side, I have no issues of NPCs being affected by time stopping. But for PCs, ideally, you'd want a situation where it's those who are supposed to be directly affected and not have any collateral victims. If you do want to do something that's large scale, it would be allowed, but an announcement to the playerbase would be needed as well as an opt-in. This could mean that not everyone in an affected area is actually frozen in time.
Hope that makes sense! If not, feel free to ask and I can try to clarify.
Soul Gems.
There are many different ways to go about these kind of items. In general, I let the players the decide how they'd like it. The options are the following:
• The item exists and just works in an optimal way. It all works or exists, but there's no need to worry about keeping a balance. An example would be vampires—in this option, they wouldn't have urges to feed on blood at all even though they still could. In your case, the soul gem would just stay clean all the time, so it would be like the "stasis" method you mentioned.
• The item exists in its full capacity, but an alternative is available. This would be what you mentioned as "world provide something else." In general, it would be a kind of direct alternative. Back to the vampires, they would just eat normal food. For metaphysical needs, such as mana, emotions would be the alternative. In your case, the use of the emotions might be the best alternative. To put it more in line with the canon, what you could do is have the character find other negative emotions and try to purify it with their own positive emotions. These negative emotions would basically replace the witches.
• The item exists, but is completely neutralized. This is a little similar to the first option, but also very different. In this case, you're effectively sealing the ability completely, so it's like a complete nerf. This should really only be used with things that, while important to the character in some respects, isn't actually necessary and its "disappearance" would just allow the character a better chance at living a normal life. This can work in your case, but I'd advise against it as it does risk altering how the characters work greatly. It would be different if the soul gem was just some external source of power and that's it, but as I understand it soul gems also help define them in a way.
So those are a couple options you could do, and you could mix it up a bit, too. One thing I'd note is that you could keep the aspect of the soul gem getting tarnished. You could this in one of three ways: 1) tarnished by the character's own negative feelings, 2) tarnished by the negative feelings of others (strong ones, of course), or 3) a mix of both. This way, you can keep a kind of dynamic if you opt to have the soul gem still working in some capacity and open up some more potential plots, be it personal or gamewide.
Phew, sorry for the long answer! But hopefully that helps.