How about we give necromancers different types of zombies to summon according to their "necromancy skill"?
Maybe necromancers could start off weak as they still haven't mastered the skill to reanimate the dead so when they start creating their first zombies those will be like classic dumb and slow waking zombies.Then,as the necromancer improves his skill over time,the zombies he reanimates become more and more lethal,going from classic Homero zombies to the more faster Zombieland-like zombies to the more smarter Land of The Dead zombies(the ones that can actually use melee weapons but in a very poorly manner) and so on until the necro becomes so powerful(this after several in-game years,mind you) that he can summon full revenants(basically a zombie that can use armor,dodge,jump and use weapons just fine)
This system could be great for both modes.In fort mode the sieges of a necromancer would progressively get more dangerous as you build a successful fort over the years.That siege may take a while to get to you but when it does get your fort you better be prepared.And in adv mode it would actually give you more challenge to your tower raids.
What do you guys think about this?
Experience-tiers for interactions/syndromes/secrets would be lovely. You really should post this as a separate suggestion.