An item that gave maximum AP, CDR, and mana regen would potentially be a better version of Morellonomicon. As such, it would become a rushed item on at least some champions, no question about it.
Well, that's assuming it was a gold efficient item. If it was as mediocre as Essence Reaver is, no one would really care. That also does kind of bring a thought to me. Morellonomicon actually represents the niche Essence Reaver was designed to fill (a good first-rush item that covers your mana cost and gives good stats), but it failed to do so. I'd actually call Essence Reaver the failed AD Morellonomicon.
Merc's isn't as well received as Zhonyas (especially since Hexdrinker exists), but I don't think you can argue that they're intended to be equivalent items. The situations they get you out of are different, but the QSS active is by no means bad or undesirable.
Zhonya's seems to be much more of a general use item than Merc's is designed to be. The stasis is a much stronger active, the stats are more efficient, and it has a better buildup. The QSS active isn't bad or undesirable, but it isn't as good as other things I could have, particularly with the gold inefficiency the item has. Zhonya's is always one of my best options. I have to acknowledge that there may have been an intention to have some equivalency between them, but I think their use cases are just too separate. That is kind of pedantic on my part though, so I guess I'll agree that you have a good point.
Ezreal is great, but he is the exception for AD carries. He has literally the most build variation. No other ADC has as much build diversity as him. Say I play Graves. Every game, I use the same item build. I can maybe throw in a brutalizer, but it's really not a good choice in the vast majority of games. For someone like Caitlyn or Jinx, there is no choice. I use the same build every game, until the fifth item. If I didn't, I would be making a mistake. I love Ezreal's build diversity too, but he's one of the only ones who has it.
I would like to see a better item selection across the board, and I think that meaningful item choices would really change the game.
Yup. Have to agree.