Well, having the Spellcaster type actually makes them combo with one of my custom cards: Rook. Rook cares if the cards played after it have more than one type. It doesn't matter with any of the official cards, as they only deal with major types and Attacks (excepting Vagrant, but that doesn't count) and I haven't seen other fan cards in my short journey through the forum that count the types or care about various arbitrarily named types.
Have you tested these much? I know you have your other fancy expansion, and have been busier than you wanted to even test that. I have some ideas off the top of my head (since this thread is my fault anyway. XD)
How about a Spellcaster card that increases or decreases the number of Spells in the Kingdom? Does casting Harm have any issue with not being an Attack? I'm not even going to suggest a Black Market variant that just scrolls though the different spells, that's so boring and I think it's been done before. Have you avoided making an Attack Spellcaster - it makes sense to me? Maybe too random, but how about a one-shot Spellcaster that uses all the spells at once or all the spells you have prepared? How about an overpay that lets you cast a spell ... no that doesn't make a lick of sense. How about an Attack Spellcaster that removes other players prepared spells if you cast that spell, like Taxman? What about a Spellcaster that has a stronger effect if you have more spells prepared - that should satisfy the level-up condition without being a Traveller or using more materials? How about a spell that is a Moat if you have it prepared... no that's just silly and doesn't even match the rest. What about one that uses the Trade Route mat to improve the power of the spell? A spell that reduces card costs; or would that not be different enough from +coins? Maybe there could be a Spellcaster that completely replaces Scout, Chancellor, or Rebuild... aaaaand now I'm rambling.
I have TTS now, so I can test stuff or just play stuff, too.
These have been tested only in the two games i described earlier. So, not much.
About Harm, we didn't play with it because my opponent thought it was too expensive. The fact it's not an attack is one of the few things it has in its favour. It's a bit like a cheaper IGG in that respect. Also, doing a wording that makes the Spellcaster an attack didn't seem worth the effort.
A Spellcaster attack that attacks in a common Dominion way seems unneccessary. Spells are complex enough as is, no need to mix something in. Spellcasters that attack Spells are useless against opponents playing without Spells, so they actively make players NOT play with Spells. Sure, i could do some "they un-prepare a Spell or gain a Curse" thing, but man, complexity again.
About changing the number of Spells in the kingdom, i think 3 is a fine number. It's cool that sometimes a Spell that goes well with a certain Spellcaster is included, and sometimes not. Also, again, using more Spells can make games too complex/confusing. Besides, some Spells, like Insight, are pretty weak, so having them isn't much different from having less. I mean, you could also have a kingdom card that increases the amount of kingdom cards, and we have. But Young Witch has a specific use for that card. You could make a Spellcaster that adds a special Spell only that card can cast, or something, but again i'm not sure that complexitiy is reasonable considering how fun it is.
A Spellcaster that allows you to use all Spells you prepared at once might work. I have something along that line with Wizard, of course, and that's allready strong. Power, too, is pretty strong. I'm not sure how games with only one of those two cards will look.
Casting out of turn order or your action phase is something i'd like to avoid, though. I once considered a card that lets other players cast a Spell. Then it became a card that lets other players prepare a Spell on gain. Then it became Wizard. There's probably some unexplored design space here, though.
A Spellcaster that does something better when you have more Spells prepared has something of a coin token card that gives you something for the oin tokens you have. It's like, you want to spend these, but you also don't want to spend these. I'm not sure how that plays, but for now, my target is to make people understand and want to play Spellcasters, which of course implies casting Spells. Mostly, i'm just not a fan of that level-up thing. It's an entire new level (ha ha) of complexity, and Spells are complex enough as is.
A Spell that counts some tokens seems nice, but i tried to go with very short tets and it wouldn't fit. It's not that complex, so it might technically be okay. It's mostly a design issue, i guess.
Glory replaces Rebuild (kind of), Insight replaces Scout. A Chancellor Spell might be fun, though. Let's call it "Chance"