Played alittlein 2013 , just picked this up again.
Reading the last few pages, it looks like a necromancer is being released in the future? That's always my favorite class (or warlock). What can I build in the mean time that's similar? I really liked the warlock's seed of corruption style of DoT spreading in WoW. It looks like I would need to aim for Contagion, correct? What about adding blasphemy to it as well?
Things to understand about the new "classes" coming if you haven't followed is they're really just subclasses, and they add a bunch of passives similar to the skill tree but specialized. Necromancer is a Witch subclass with a focus on pet buffing stuff, but it won't change the current builds all that much, just make them better. If you want to play a summoner you can play one right now and, barring new skills, it'll play more or less exactly like Necromancer would, just less powerful(but obviously the game's balanced with the current power levels in mind, I assume Ascendency will rebalance most shit to take into account the power creep from subclasses).
For example Necromancer adds chaos clouds remaining when minions die, stronger spectres stats, reduced damage taken based on how many zombies you have and make the offering minion buff spells affect you also(which is pretty neat since they're strong buffs). Those are nice but it's not like new minions, new mechanics or an actual new class on the tree, it's just a witch focused on minion stuff.
As for the "future", it's in 3+months at least, so don't expect it soon either. It comes with the next expansion, probably a bunch of other stuff including the new daily labyrinth thing you get your subclass in.
So if you want to play summoner, you can do that. They also added chaos spells in this patch so you can do that, however it's not really like seed in wow. Seed kinda behaves the same as explosive arrow, you put a bunch of arrows on stuff and they explode on everything around, at least that's how I remember corrupting seed. The new chaos spells are really just dots, like a single target dot and an aoe dot that spreads the single target dot at the same time. Mixing stuff is possible but as usual it can be problematic due to straining yourself thin on the tree.
As for leagues, since a new league started, you probably want to play that, Talisman league, Standard and Hardcore are probably super dead at the moment. Unlike previous leagues, this one's the same for both hardcore and softcore(well minor differences in item drops and such), so you pick whatever you prefer, probably more people playing Talisman softcore though.