Well i don't really like the ranger as is written, but the Ranger idea definitely fits perfectly into this setting. Bards would fit nice, but i think their spell lists should be replaced with more bard-like abilities(such as their countering abilities, and fascination powers).
Druids definitely are cool and will definitely be VERY pissed off!!

Are you thinking of creating a mechanic for this nature-beseechment requirement. You have to make a Diplomacy check or something with a particular aspect of nature (earth, water, ice, etc.). Or maybe use their knowledge nature skill instead? With th DC equal to 15 + Spell Level. Also maybe a bonus if its essence is a dominent point of the area (ie a bonus to casting fire magics if near a volcano, that kinda thing)
Paladins don't really fit the setting, especially since they don't have a patron anymore. The revival of such a class might make for a cool PC quest, though!
I have the same problem with monks for my homebrew, i have an "Orient" type area in it, but the Monk class just doesn't seem to fit well. I can't even put my finger on why, though. Too mystical maybe.
I have a idea on how to allow elves in your campaign, but with some drawbacks. Say that the Magical Fallout sterilized the race, so any characters made would have to be at least Middle-aged(PHB page 93), complete with physical stat loss. Their physical traits would now read -1 Str, +1 Dex, -3 Con, and their Mental stats would be +1 each. Although you might want to give them a Knowledge, Craft, or Profession skill for free at several ranks, to acknowledge their longer life.