Magic in RP

One of the most interesting, and maybe the hardest thing I face in an RP is the magic system.
When I use the term "Magic System", I speak about every system or mechanic that give the player the freedom of doing whatever they imagine. (Hard to explain so I'll give an example)
Let's say, our characters can use alchemy. They can make almost anything they want, as long as it follows the rules of alchemy(equivalent exchange for example). So my character can, for example, take some iron and make a sword, while another character can make a machine gun.

Now for the question, how do you think(both as a GM and a player) such mechanic should be used in an RP, should it be constrained or open to imagination. For example having a magic system with a set of spells the players can use(Like in the Harry Potter books), or based on imagination of the players and by that, giving them full control over it(Like in the above mentioned example, which is based on Alchemy[FMA in particular]).

It depends on the type of "magic" on the universe, per se. For example, Noble Magic allows you to some kinds of types of magic (Elemental, Life, Force, Death, and I forgot the other one.) However, how you use these type of magic is certainly up to your imagination (like how I planned my character's Death Magic to mimic the effects of Elemental and Force magic.)

However, in universes like Harry Potter where there are certain combination of spells that will create a really specific effect, having the magic up to the imagination of the player might prove to be a little chaotic since you need to keep track of all the new spells just to keep the canonity of the RP.

It all depends on the setting, really.

I think that players using magic can be both a good and bad thing, but it entirely depends on the players that you are dealing with. Specifically, whether the players want to make a story, or beat the game.

I think that here on PC, most of us are more interested in writing a story than "winning" the RP. When it comes to writing a story, we are more likely to use magic in such a way that makes the story more interesting, not so much breaking it.

But I have played with people in the past who don't care about the story but more about beating the bad guy at the end of it. A lot of D&D players are like this I have found, and care more about breaking the game than experiencing it. There is nothing wrong with that, but if they push the abilities they are given to the limit to the point where they break the game, that's where problems arise.

When it comes to magic, it all depends on why the people are there playing. If you are more interested in the story, you are more likely to have your character accidentally shoot a magic missile into their own leg rather than having it set off a Rue Goldberg chain reaction to kill the big baddie a hundred posts too early.

As with anything in an RP, there should be limits. However, you should also allow players to push those limits, and if they have enough imagination, expand the limits.

For example, say an RP gives a list of basic spells, and characters know a small number of these spells. The player then creates an additional spell the character knows, with the GM having final say whether the spell is acceptable. This keeps magic under control, with players not allowed devastating super spells. However, they are encouraged to use their spells however they want, and half the fun is in figuring out how a spell will react in a certain situation. In addition, players can create new spells during the game, again with the GM having final say. This requires a GM who allows varying levels of realism and physics in their game, but creates an atmosphere of fun and excitement that rewards players for thinking outside the box.

After all, if we only know certain spells and they only react one way, we're just playing Pokemon (or a similar RPG). But if we can all do anything we can imagine, we're just the Green Lantern Corps. Both can be fun, but by finding a stable middle ground we create a balanced system that can appeal to anyone.