I don't know what could be done to reduce the complexity of spellcasters in 5e. Currently the number of spells prepared is a lot smaller than in previous editions, so they are already less complex, except the Wizard due to casting Rituals from all known spells. Personally I would also limit the number of known spells to Clerics and Druids, to the same numbers as Wizards. But this is a no-go for a lot of gamers, so it won't be done.
I have to say, I'm mightily impressed they had the stones to reduce the number of daily spells as much as they did. Here are some comparisons:
OD&D (Greyhawk Supplement)
[TABLE="width: 500"][TR][TD="align: center"]
Spell Levels[/TD][TD="align: center"]
1[/TD][TD="align: center"]
2[/TD][TD="align: center"]
3[/TD][TD="align: center"]
4[/TD][TD="align: center"]
5[/TD][TD="align: center"]
6[/TD][TD="align: center"]
7[/TD][TD="align: center"]
8[/TD][TD="align: center"]
9[/TD][/TR][TR][TD="align: center"]Level 20[/TD][TD="align: center"]7[/TD][TD="align: center"]7[/TD][TD="align: center"]7[/TD][TD="align: center"]7[/TD][TD="align: center"]7[/TD][TD="align: center"]7[/TD][TD="align: center"]3[/TD][TD="align: center"]3[/TD][TD="align: center"]2[/TD][/TR][/TABLE]Total spells per day: 50
BECMI
[TABLE="width: 500"][TR][TD="align: center"]
Spell Levels[/TD][TD="align: center"]
1[/TD][TD="align: center"]
2[/TD][TD="align: center"]
3[/TD][TD="align: center"]
4[/TD][TD="align: center"]
5[/TD][TD="align: center"]
6[/TD][TD="align: center"]
7[/TD][TD="align: center"]
8[/TD][TD="align: center"]
9[/TD][/TR][TR][TD="align: center"]Level 20[/TD][TD="align: center"]6[/TD][TD="align: center"]5[/TD][TD="align: center"]5[/TD][TD="align: center"]5[/TD][TD="align: center"]4[/TD][TD="align: center"]4[/TD][TD="align: center"]3[/TD][TD="align: center"]2[/TD][TD="align: center"]-[/TD][/TR][/TABLE]Total spells per day: 34
AD&D
[TABLE="width: 500"][TR][TD="align: center"]
Spell Levels[/TD][TD="align: center"]
1[/TD][TD="align: center"]
2[/TD][TD="align: center"]
3[/TD][TD="align: center"]
4[/TD][TD="align: center"]
5[/TD][TD="align: center"]
6[/TD][TD="align: center"]
7[/TD][TD="align: center"]
8[/TD][TD="align: center"]
9[/TD][/TR][TR][TD="align: center"]Level 20[/TD][TD="align: center"]5[/TD][TD="align: center"]5[/TD][TD="align: center"]5[/TD][TD="align: center"]5[/TD][TD="align: center"]5[/TD][TD="align: center"]4[/TD][TD="align: center"]3[/TD][TD="align: center"]3[/TD][TD="align: center"]2[/TD][/TR][/TABLE]Total spells per day: 37
3rd Edition (Int 18)
[TABLE="width: 500"][TR][TD="align: center"]
Spell Levels[/TD][TD="align: center"]
1[/TD][TD="align: center"]
2[/TD][TD="align: center"]
3[/TD][TD="align: center"]
4[/TD][TD="align: center"]
5[/TD][TD="align: center"]
6[/TD][TD="align: center"]
7[/TD][TD="align: center"]
8[/TD][TD="align: center"]
9[/TD][/TR][TR][TD="align: center"]Level 20[/TD][TD="align: center"]5[/TD][TD="align: center"]5[/TD][TD="align: center"]5[/TD][TD="align: center"]5[/TD][TD="align: center"]4[/TD][TD="align: center"]4[/TD][TD="align: center"]4[/TD][TD="align: center"]4[/TD][TD="align: center"]4[/TD][/TR][/TABLE]Total spells per day: 40 (plus 4 0-level spells)
5th Edition (with Arcane Recovery)
[TABLE="width: 500"][TR][TD="align: center"]
Spell Levels[/TD][TD="align: center"]
1[/TD][TD="align: center"]
2[/TD][TD="align: center"]
3[/TD][TD="align: center"]
4[/TD][TD="align: center"]
5[/TD][TD="align: center"]
6[/TD][TD="align: center"]
7[/TD][TD="align: center"]
8[/TD][TD="align: center"]
9[/TD][/TR][TR][TD="align: center"]Level 20[/TD][TD="align: center"]5[/TD][TD="align: center"]4[/TD][TD="align: center"]4[/TD][TD="align: center"]3[/TD][TD="align: center"]2[/TD][TD="align: center"]1[/TD][TD="align: center"]1[/TD][TD="align: center"]1[/TD][TD="align: center"]1[/TD][/TR][/TABLE]Total spells per day: 22 (plus 3 cantrips)
5e mages are a little more front loaded than their TSR-era counterparts, but sacrifice high-end versatility, to say nothing of raw power. Which is pretty much the common sense thing if you want to reduce wizard quadracity.
One way they could reduce complexity of Mages is to restrict them to spells in their tradition, with optional complexity coming either from a generalist Arcane Tradition, or a feat that allows cross-school spell acquisition.