The lovely thing about writing fantasy is that you can make your world and your characters anything you want them to be. This is the reason I love writing it.
However, I'm acutely aware that these 'magical' worlds must have some rules for them to function. E.g. where's the 'reality' - if you will - if I made a character who could shoot lightning bolts to their heart's content and put them up against someone who could only use melee. Obviously lightning-girl/guy is going to win hands down.
My rule would be some sort of cool-down or a tiredness factor (you can tell I used to be a gamer). Something to make it seem a bit more 'real'.
I have many rules in my books. They're in place to make it harder for the characters to just kick-ass with no questions asked.
Rules can be character rules, like the one I just mentioned, or world rules like 'this power can only be used in certain places'.
I think it's important to establish the rules ASAP. There's nothing I hate more than a writer (and I see this more in TV shows than anything else) creating a really powerful bad-guy and then 2 seasons down the line, having the MC 'discover' an even more powerful widget to kill him/her.
What do you all think? Do you have rules too?
So these seem like magic rules specifically. For fantasy, the key, as someone else has said is to make sure you have elements of the fantastical. However, for general world-building, there need to be world rules, language rules, and magic rules. Your fantasy world can be whatever you want it to be, which is always fun, but your world has to make sense.
Even in worlds where you use soft magic systems (magic that doesn't have strict cause and effect, science-like rules), they have vague rules, and the soft magic is there to make things more mysterious and mystical. So even in the worlds with magic that isn't as scientific as most modern fantasy, the magic has rules, and is kept to a certain power level to avoid it feeling like magic can solve all problems.
Personally I believe there is one rule to fantasy writing: It contains elements of the fantastic.
That's it.
From there there are styles, or subgenres that have commonality. Like your D&D rules and stock characters, or strict rule magic systems like Sanderson's Allomancy.
But such is not the only way, and I would caution against believing this is definitive of genre. It is an option and if you wish to write something derivative, go for it though make sure you make it your own. Just don't let it hem you in.
In ur example Debbie I would argue that Melee would arrange an ambush situation, narrow geographical area, lots of water. Lighting would risk frying herself and companions.
Also, are your in-world rules pertinent to the story? Perhaps how the situ arose is irrelevant, the reader must accept that this is the world, and the charas reactions are what the story is about.
Robert, you inspired by Jim Butcher, Ben Aaronovitch?
I write all of my stories in this genre and, yes, I agree: there have to be rules. One of my characters was recently promoted to Sorcerer-hood by inheriting some of his mentor's powers, which came with a whole raft of issues, not least of which was that he now has enough power to destroy a small city, so how on Earth do I put limits on it so that he can't?
I wrote a few core rules for magic in this world, which loosely are:
1) Magic can cost you your life. The more powerful the spell, the more magic you use, the less magic you have to keep you young. The older you are, the more dangerous it becomes to cast big, falshy spells. (Magic-users in this universe can live for 400 years if they control their magic well. It gives age-defying properties to its bearer).
2) Robes can block spells. They're more than a fashion statement - they're magical armor.
3) Spells can backfire if not cast accurately.
4) Magical solutions to everyday problems do not always have the intended effect. For example, although in this universe trolls were captured by magic uploaded to the internet by the magical branch of the government to make travelling in rural areas with lots of bridges safer, they now make a nuisance of themselves on blogs and chat rooms.
5) Magic cannot influence computers.
6) The magical world's government is, to put it mildly, inept.
7) Magic was forced underground during the witch hunts of the 16th century so cannot be used openly.
There are lots of other rules, but these are the core ones.
Sooner or later, the MC will have to develop a powerful widget in this story to deal with what he's dealing with, but the hoops he has to jump through to obtain permission to use his full powers are often not worth the bother.