A few ideas of how to save D3

[Elric of Grans]

There are other synergies. For example, Passives that give a bonus under X condition. If you have an ally who can create that condition for you, then you can free up skill slots for other purposes. For example, a Demon Hunter could get their Cull the Weak bonus from a Blizzard, allowing them to pick high-damage attacks over Slow-inducing ones. Definitely not the same as DII, but there is plenty of room for a party to create builds that complement each other.