Is mana even a resource?

As time goes on, more and more champions come out without mana, and sometimes with another "resource" instead (though sometimes its just a glorified CD ). The thing is, I don't think this is a bad idea because so few champions have ever really needed to manage their mana beyond "Do I have enough mana to use my combo right now?". Right now mana seems to be nothing more than a way to control a champion's power curve throughout the game. Early game you are restricted by your low mana pool and/or high mana costs that prevents you from spamming your abilities. Eventually though, as time goes on, eventually you will get to a point where your abilities have almost insignificant costs because either your abilities cost next to nothing, or your natural mana bar has gotten big enough naturally or from some item that your abilities barely dent it. Either way, the only reason you will ever run out of mana is because you haven't gone back to base in a while and probably should have to buy more items.

In comparison, some of the newer resource systems require actual management (for optimal play at least) and have much more interesting tactical decision making involved. Lets take Gnar for example. Here is champion who has a fury system wherein if it maxes out, he transforms and gets a different skill set and different stats. This creates an interesting dynamic because now the player controlling Gnar needs to be aware of how much time in combat he spends in order to control whether he stays in mobile ranged mode or go into tanky CC mode. When he gets close and is about to transform, both the Gnar player and his opponent have to pay attention to when that moment will occur and what to do with it. Should the Gnar go all in? Should the opponent back off preemptively and wait for the transformation to run out? Should the opponent try to keep Gnar in combat and force the transformation? There's actual game play there and both players (and then later on both teams) need to manage Gnar's fury resource. Mana simply doesn't do the same thing

Now I'm not saying we should just remove mana outright. I think there could be some real potential to having a universal resource that functions differently with each champion. For example, what if marksmen used "ammo" instead of mana, and each "bullet" was equivalent to X amount of mana. When they use abilities, it costs some number of bullets to use them but then every marksmen could also get a bullet back for each minion they kill.

Or in a case by case scenario, a champ like Kass could have a charge bar that filled up when abilities were used around him (with maybe increasing effect when used by other champions instead of himself) and that charge bar could determine how far he could jump with his ult.

Would it be worth it? Are there any champions that could put a cool twist on this?

(Also do note that I'm talking more game play and less game balance)