The Guardian already specializes in summoning.  I think hypnotism works fine for the wizard.  since the mana cost is on the high side though, I think the sovereign path should include something that increases mana.  Maybe +1 mana per resource.  Also, maybe the sovereign could start with two spell casts, or bundle that with a +mana skill to show a focus on spells.  Something that would encourage you to select that first.  I find myself ignoring the +spell cast skills with all sovereigns until late game.  Bundling them with something else for the Wizard would be a nice change from other sovereigns (at least for the first one).  

Maybe the + spell cast skills can come with buildings that could be built that increase mana. Maybe a Scrying pool that creates +1 mana per city size.  or +1 per connected crystal. 

Which reminds me, the cities don't show how many resources are connected to them.  There are buidings that increase skill per connected shard, for example, but the connected shards are not "closest".  But seem to be "closest to the city that existed when the shard was connected".  Would be nice if the city screen showed how many shards, or other resources were connected so you didn't have to click on them all to see if it is worth building a +skill building.  (or I could simply not be seeing this info, and it is already there...)

I'm still not seeing why everyone loves coercion.

In all my insane games, my winning strategies have always involved hypnotism or chant of vitality.

It really levels the playing field when you can hypnotize some of the best SK units. Mainly warg riders. I usually go actively hunting for warg riders or shadow wargs if I can't find the riders. It's a great skill and is very powerful as it allows you to build a massive army without cutting into any of your limited resources.

Agree totally on the +spell cast skill. Having them give some other bonus along with +cast for the wizard would help the wizard's skill tree to stand out a bit more. I'd also suggest a skill that increases the effective number of shards for spell-casting. So if you had 2 shards and cast a fireball, it'd be like you were casting a fireball with 3 shards or something like that.