Personally, I would recommend you just take some time to look through the various magic rules and systems presented across ICRPG’s publication history and treat the entire oeuvre as an a la carte menu, a smorgasbord from which you can cherry-pick just the items that appeal to and work for you and your table.
In my own game:
I keep the simple, elegant roll-to-cast system that is the core of ICRPG magic.
I differentiate between arcane magic (INT spells) and spirit magic (WIS spells); INT spells take up inventory slots (except for Mages with Spell Scholar mastery) and have casting constant, while WIS spells do not (and they remain intrinsic to the caster once learned).
I do not track spell school mastery as presented originally in ICRPG Magic, but I do note the schools for flavor and let them work mechanically in other ways.
I permit players to choose spells from Core 2.0, Master Edition, ICRPG Magic, or even Khan’s spellbooks. I even encourage them to “roll their own,” and then I work with them to assign a reasonable tier and specification.
I note the “tier level” of any arcane (INT) spell and figure that into its casting cost; most of the spells in the basic list (Master Edition, pp. 46-47) I consider to be “tier 1” but some I redesignate at higher tiers respecting their scope (see spell creation rules, p. 321).
I use the POWER level rules for arcane (INT) spells (only). Any given cast can be as written (POWER 1) or augmented to increase magical effort, duration, or other aspects, depending upon the specifics of the spell (POWER 2+). Maximum POWER on any cast is 4.
Casting cost for INT spells equals (TIER) x (POWER) in hit points —because stun points are for wusses (and superheroes)…
An arcane caster who drops to 0 HP or below as a result of casting costs is rendered unconscious but stable—NOT dying, unless the let loss results in negative hit points greater than the character’s maximum HP (which results in some Last Jedi sh!t…).
Nat 1s demand a Failure roll on the Mercurial Mishaps table (p. 320).
I definitely allow players to build Mage characters using the “advanced Magic” character construction rules (ME, p. 322), but I do not allow them to hybridize the build between the advanced system and the standard rules in the Player’s Guide section—it’s one of the other.
That’s about it—that’s magic at my ICRPG table. Your system will be your own. Good luck.
I do roll to cast with loot items attached to the spells: masks, hour glasses, rune emblazoned skins, candles, etc. Some spells, usually attack spells, have limited uses, like magic missile is a bag of troll’s teeth. Once you’re out of teeth, no more magic missile. Dedicated casters do get between one and three spells, and these are the ones that lead to magical mishaps on the nat 1 casting roll. I also keep very tight control over straight damage and healing spells. I did come up with a usage die system, but didn’t implement it. I just don’t feel like points of any kind feels very magical.
Great question! You’ve already gotten some wonderful answers above. Those three are very experienced gamers and have good advice - I would heed it. And since this is a forum, I’ll add my take, too. Here is what I use:
Ancient Sorcery: Vancian-style spells that have been perfected by wizards of old to avoid mercurial failures. They are very taxing, however, and can only be cast a few times before resting. I usually limit to once or twice. Just “burn the slot” and you’re good - no roll to cast, no chance of critical failure. These are rare, and highly sought after. I would reserve these for quest rewards (“Do this thing for MERLIN, and he will teach you one ancient sorcery”).
Modern Magic: Roll to cast. These are newer spells (last few hundred years or so) that are volatile and unpredictable. They don’t always work, they can critically fail (mercurial magic), and they can critically succeed (nat20). Since these are not as taxing as the old sorceries, they can be cast over and over as many times as the caster wants to risk critical failure.
Mercurial Magic: Critical failures suck. I took inspiration from DCC, if you’re familiar. Casters naturally limit themselves because they don’t want to die, mutate, kill a friend, or open a gateway to… other places. Among other things.
You might also want to check some of the discussions we have already had on magic, here: