I'm not sure how they'd differentiate between Necromancer poison/disease and fungal disease. Maybe the Necro spreads diseases by virus while this other mage spreads diseases through living fungal parasites?
Well, maybe instead of the diseases directly hurting enemy creatures, fungal diseases allow the D/N mage to summon more troops (for every point of damage the enemy creature takes, 1 very small friendly creature is spawned in that zone), change the terrain (if a creature dies, it turns the zone into a mushroom forest or something), or buff friendly units (every fungal creature that attacks the cursed enemy heals based on damage done). A Dark/Nature mage wouldn't even necessarily have to specialize in diseases, and could focus on something else entirely.
You don't understand.... I just really want Mushroom People in Mage Wars. I don't care how or why. I want them almost as badly as I want people riding giant seahorses and a sharktopus (as mentioned by Ozmo). [Include them in PvS, devs! You know you want to!!!] Don't ruin my dreams
! It would be nice to see Mage Wars occasionally embracing some more humorous or unusual concepts.
Also there is already a thread on the subject of joke expansions:
http://forum.arcanewonders.com/index.php?topic=13081
And the lawyer idea was listed in the very first post too. I guess I need to become more creative...