A lot of newer OSR systems (most famously Bastards, I think?) allow you to create spells by combining verbs and subjects - e.g. Speak with + Stone, Become + Fire. It's a cool system! Not intrinsically better than D&D's typical pre-written spell selection, but definitely bringing its own unique strengths.
In this blog post I wanted to spitball some things I would do with a modular magic system which I think would be fun.
Note: for the purposes of the following discussion, assume that each given magic word (not pair; word!) can be used once a day.
Rearranging spells on-the-fly
So I think the assumption with most instances of modular magic is that once you roll the components of a spell and jam them together, they're kind of set in stone. I know in Bastards you can rearrange your words when you level up, but for the most part they're static.
This is perfectly fine, but I think it would add some tasty strategic depth if wizards had the option of taking their spells apart and rearranging them more frequently.
This adds a variety of gameplay wrinkles. Say a given wizard has the Verbs Connect, Become, Enlarge and the Subjects Flesh, Emotion, Fire. At the start of the day they have to try and predict the coming challenges and decide what would be most useful to them.
Should I pair Connect and Flesh so I can heal my friends? Or should Flesh go with Enlarge so I can make my buddy Thurg bigger? Wait, but I also want to Enlarge Fire so I can make torches explode. Hang on - what would happen if I Enlarged Emotion...?
This already is so much more deep and engaging than simply picking what spells you have prepared from a list. And it gets more interesting if you allow Wizards to rearrange their spells during the adventuring day, but at a cost - e.g. spending a Turn in the dungeon to rearrange conjures a lovely mental image of a wizard frantically working arcance kludge by sputtering candle-light as his allies watch the doors, knowing an orc patrol could walk by at any moment.
What if we up the tension even more? A wizard can rearrange his spells instantaneously, in the midst of combat even, but doing so this rapidly and carelessly risks a catastrophic mishap.
Wow, this ambush is going badly. Enlarge Fire would be really nice right about now, but Enlarge is tied to Emotion. I could cast Enlarge Fire anyway, but... do I want to risk blowing myself up?
Rearranging spells is complemented nicely by a Mausritter-style item card inventory system. You can have a card for each magic word and just join them into pairs using a paperclip or Blu Tack. Easy peasy!
The final thing that I think makes this cool is that I would feel a bit like an actual wizard tinkering with magic and experimenting, rather than just picking premade things off a list. Which would be neat!
This is kind of by-the-by and not really about modular magic per se, but I kind of prefer magic systems where you can use each spell once a day, rather than having an interchangeable fuel like spell slots that you could theoretically spend entirely on one spell over and over. Most tables I've been at have spellcasters that spam fireball over and over, which is BORING. Making every spell single-use means you have to actually adapt as the adventuring day proceeds and can't do the same thing over and over.
Enchanting Objects
So obviously rules for creating magic items have always been part of D&D, but I've just never found them satisfying. They're somehow simultaneously fiddly and vague? Not providing sufficient depth or interest but also being a bit of a headache to use. Bah.
Here's something much cooler: a Wizard can spend one week implanting a Verb and a Subject into a mundane object, rendering it magical. For example, they could jam Connect Mind into a pair of rings that allow the wearers to communicate telepathically, or they could imbue a bottle with Conjure Water so it produces infinite water.
This replaces a boring check and balance (how much gold will this cost?) with a fun one (what if I need these magic words for something else?). You're essentially making your spellcasting slightly less flexible in return for creating a more persistent effect that your allies can make use of, which is a cool trade-off to think about.
We can also do the same thing, but in reverse. A wizard can spend a week destroying a magic item in such a way as to extract some of its power in the form of a magic word. Take the classic flaming sword as an example. It contains the spell Conjure Fire. A wizard could break it down and gain either the Verb Conjure or the Subject Fire. I would probably only let them pick one or the other, or else this could possibly be a bit too strong...
Wizards vs Sorcerers
So all this chopping and changing and tinkering is all very well for the scholarly and technical wizard, but what of sorcerers? Their magic is a part of them, not a spare part to be picked up, rearranged, used or discarded.
In this system, each sorcerer has a single magic word entwined with the very essence of their souls - their Quiddity. A sorcerer's Quiddity could be a Verb (e.g. Invert, Combine, Steal) or it could be a Subject (e.g. Beast, Appearance, Fire).
Sorcerers can only cast spells that contain their Quiddity, but they can use their Quiddity as much as they like (rather than once a day like normal magic words). If that's Fire then they do stuff with fire, and that's it. This obviously makes them a bit inflexible compared to wizards, but it's fantastic if you really like using fire (a wizard with the word Fire would only be able to use it once per day). Also, I'm picturing magic words as taking up an inventory slot each, but a Sorcerer's Quiddity obviously would not count towards encumbrance.
I've not thought too deeply about it, but I would probably make Sorceror magic a bit more potent than Wizard magic (e.g. affecting more targets, doing more damage, lasting longer) to compensate for the decreased flexibility.