Hiya!
Crazy powerful? Ok, change it. Do you honestly think these "professional game designers" get everything right the first time? Hellz no! I'm sure that some playtesting of the Elementalist I whipped up would reveal problems, oversights, or other unforeseen quirks. I'd just play it and see what proved to be too powerful or too weak, then do some rewriting. Save. Hit print. Hand to player. I mean, look at what they actually produced, printed, and call 5e D&D. How many threads are out there claiming "Class Feature 17 is overpowered", or "Subclass B is super-weak", etc...etc...etc. See? These "professional designers" got stuff 'wrong' anyway...why do you place more faith in what they write versus what you (or any other DM) write?
Spells an such...an easy thing to handle, really. You could spend time going through the list to designate what spells fall under what element. Personally, I'd just do it through play and through player asking "Hey, what does
Cloud of Daggers fall under?" (btw, I'd put it under air with a bit of flavourfull fluff, but an argument could be made for earth, with a bit of flavourfull fluff again).
Shatter - Air again.
Cone of Cold - Again, Air. Really, I may be anwering "Is this a water spell?" ten times in a session, but probably for only a session or two. Then I never have to answer it
EVER AGAIN. In my case, "ever again" would equate to somewhere around 34+ years. So...two sessions of 'on the fly work', or maybe another half-hour to hour of spell checking, for a 34+ year pay off? Sounds like a pretty sweet ratio of "work to fun" to me!
I asked in another thread I started about "Aversion to Creation"; your last two sentences are a prime example of why I felt the need to ask that question.

As far as I'm concerned there is really no such thing as a "professional game designer". There are only gamers who get paid to write their stuff down, and those who don't. That's the
only difference as far as I can tell. Who does better stuff? That's like art; each person has different tastes, so none is really better than another. For example, I loved Gary Gygax's writing style for adventures and campaign world (Greyhawk)...and I'm not that keen on Ed Greenwoods writing style for adventures and campaign world (Forgotten Realms). Neither of which, btw, were professional game designers when they started. They were just gamers who wrote stuff down and got paid for it.
^_^
Paul L. Ming