Hell yes! I do think spells should have a cool down or limited number of uses before being swapped with a different spell to prevent (me specifically) people from spamming snowball, but this is a great idea and I am excited to see how far you can take it.

Recently, I was on a website for DOS games and such, and ran across Mario Typing. It was fun for a bit, but you're absolutely right; it was made to teach. I love your idea to use typing as a fun mechanic rather than a learning method (though that has its place too, of course).
1) Aaaaand of course my story-oriented brain immediately created a goofy tale of a mousy secretary transported to a D&D-type world with only her keyboard/typewriter which is now magical, and she, along with the party of friends she gathers along the way, has to fight the Big Bad and his minions to save the kingdom, get home etc etc. :D
2) Your choice of awesome and very epic music fit perfectly with my story, haha. I enjoyed the simple, clean, well-done graphics too.
3) Maybe just me, but I found the font for the spells and in the textbox a bit hard to read. I would've liked both a tad larger. And/or a little more spacing between the letters? Or possibly it was the all-caps? Or again, possibly just me.
4) Different typing scenarios jumped to mind too, to change up the gameplay. (Which you've probably already thought of.) Like in Mario Typing, the faster you type, the faster the character runs--great for an escape scene. Or type to dodge a series of falling rocks, for example. And for a stealth scene, have a longer timer but if you hit the wrong key at any time, that's basically a twig cracking underfoot--too many of those mistakes and your cover is blown.
5) I agree with a previous poster that a spell cool-down would be more balanced. Other than that, I thought the choice of spells was fun, with some variety for strategy. (And yes, I did attempt to copy-paste, just to see if I could, you know, and was happy to find that I couldn't.)
6) I'll shut up now and let you get back to dev-ing! Good luck!