I just find that my spell bar gets way to cluttered with each slot being it's own entry. Makes the spell slots feel more like charges than it does actual tiers of power. Spells Should be like the Hex spell, it shows a box after you click the spell to choose what ability to target. I dont think it would be much of a leap to be able to click on a spell and then select the level you want from a similar drop down/pop up
Had a similar thought and think this sounds like the simplest as well as effective UI. Having the same spell multiple times in the hotbar but without any visual indication of which one is the higher level spell means you have to hover over the spell, wait for the pop up (I know it's relatively quick, but doing that a hundred times over adds) and then determine if that's the level you want to cast at.
level 1 spells look like they do, level 2 spells have II in them.
As for clutter I sort and seperate all my spells for quick use, ones I use are nearest the left side of the screen on top. because I do this I have no problem knowing which is which and if I oops, I oops. Greying out, or completely removing spells from hotbar means that if you pick up a throwable it's going to take that slot more then likely. So it is now in the wrong spot and is causing clutter, which I then have to remove, and put in it's proper place.
Poping up means I now have to click twice, when once it was just a single click. Five fights thats ten extra clicks. 10 fights 20. why would I want to click that many times? My poor mouse is already over loaded do to what I do with inventory, and hotbar.
Hotbar itself = alot of my personal issues with the hotbar was resolved when I switched my resolution. Now I can play with the hotbar maxed on size, and it keeps everything where it's spose to be, as I have placed it. Yes mages get alot of spells, scrolls etc. but thats mages. It's just like in pnp were you keep track of all your spells, and do extra research, make extra notes. Mages by very design are spose to be 'extra' in every thing they do, because of their spells and such.
After switching the resolution, I find the hot bars to be far easier, and less cluttered then other games that have hotbars. Far more then MMO's for instant WoW which you can fill up to six I believe hotbars with just crap. You have the option and it's very easy to do to set the hotbars the way you want them to be set up. Thats logical for you, thats really not clutter, it just takes a bit of time. Also if you say switch out your fog spell with something else, the something else replaces the spot for your fog spell. If you have random spells next to potions, in between scrolls thats on you the player, to sort the clutter. Its standard on every game, with hotbars, you gotta do a little extra to have the joys of melting faces, or shocking scum.
As I've told several players I've played with (mostly because I'm a dick) who were always going on about being unable to find stuff. 'Organize your shit. Clutter gone. Shouldn't take long to know witch bolt lvl one is in third slot, witch bolt lvl 2 is in fourth slot. Both are between Mage Armor which is in the second slot, and Firebolt which is in 5th slot. Health potions are in the first slot. Sleep is in 6th slot.
or did you want my Warlock slots without me looking at them as well. thats not clutter it's organized. most times I don't even look at my hotbars since I changed my resolution, and organized them.
but thats my 2 coppers