How are you making the determination on this "40%" number regarding spells? I am fairly certain that your first and second points can be modded already. Third... well, many high/superior quality models can be farmed from nwvault.ign.com.
I'm not sure about 40%, but I'm going to assume that the majority of it involves global enchantments, combat enchantments, some city specific enchantments, and some unit enchantments. We haven't seen an example of any globals, yet, so until we do nobody is really going to be able to put one in. I also don't see a way to change what terrain does. Elemental has its basis for special terrain types like fertile land, but MOM uses just straight up terrain type for its base, so we need to know how to generate food and production based off grass fields, forests, mountains, etc.
As to unit abilities, there does not seem to be a way to have passive on-hit, or on engagement abilities from what I've seen so far. Also doesn't seem to be a way to add magical effects to weapons. Or a way to transfer weapons/items without the use of hero-to-hero trade, or buying from a market place.
How would we also mod in a wizard tower, summoning circle, remove roaming random heroes from the playing field, introduce a secondary plane, add in an "explore" option for dungeons, and so much more I can think of.
And I think the biggest question is ... how do we get the AI to actually use this stuff once it is in?
Once they expose more of the internal games we can start looking at a total conversion, right now I'll be happy if we can get unit specific abilities in (like unit specific mana regen, not a global mana regen change.) And definitely some new spells, because the only reason to have all schools of magic is because of how rare shards can become in any given game.
