The reason and need to do this is simple---- dismantle the current unbalanced and lack-of-lore/background classes, and bring ESO back to the good old ES game fashion.
So to start with, we should keep the current stam weapon skill lines, and possbily the world skill lines, pvp skill lines and crafting skill lines.
However we should remove the class skill lines, instead bring back the old 8 types of magic---each with their own skill line. And while corresponding staves are still used to provide enhancement of certain type of magic, it is no longer a must. (However as restriction/limitation most of the abilities should be make viable only when enhanced by staves)
Other than that, if you want to learn some special skills, you need to go to certain places and join in their rank/ gain their trust:
Fighter guild provides the training of weapons, and some skills against daedric powers;
Mage guild provides general types of magic training, and perhaps provide the making of scrolls.
Undaunted provides their special ways of fighting that focus on provide grp utility;
Natural magic? Go to wyrdresses or druids;
Holy magic? churches and temples, and possibly hists;
Sneaking in plain sight? Thief guild!
wait in shadow for the one oppertunity to decide the win or loss in one hit? Darkbrotherhood welcome you
Ancient yokudan way of fighting? seek out sword schools in hammerfell
Special Arcane? summerset is full of them, or perhaps some telvanni mages?
.....Want to learn to fist fight? Well go to the place we don't talk about

So basically instead of class+weapon+special(guild/world etc) system, change it to weapon/spell+special, simpler, while all skills are accessible to all characters, there are still choices to be made. A vampire shouldn't be hearing the call from stendarr, a man of wild forest will turn down the best teacher of mind-enslaving arcane, a student of yokudan way would rather die than touching any necromancing magic, tho they study together but your restoring professor will not teach you the best of his spells if he knows you are practicing mass destruction spells as well....While the roles (tank/dps/heal) will still remain, the possibilities and combinations to become best in those roles will be near infinite and based on variety of situations, it is impossible to minmax, thus balancing the game, and giving ppl more choices and reach the real "play how you want"