Preamble: my knowledge of FFH lore is limited, and I'm going to propose summoning spells even though Kael supposedly said Dimensional won't have them.
I've read through the thread and thought hard about what kind of spells I would like to have in the game. I've come up with the following guidelines first of all:
General guidelines:
Non-specificity: spells should apply to as broad a range of situations as possible. Thus spells that explicitly counter enemy summoning and nothing else are very useless, spells that only help during the siege of an enemy city are somewhat useless, and spells that damage enemy units are useful.
Feasibility: I will try to describe spells which, according to my limited understanding of the SDK, should be possible to implement without major coding efforts. I'm pretty sure this is where I will fail.
Coherence: especially in coming up with these guidelines I tried to look at how the team has done things so far and I tried to stick to that pattern.
SORCERY should be about directly interacting with the map or the enemy. Teleporting to another tile is interacting with the map.
SUMMONING should always be about creating a new unit. I disagree that dimensional summoning should be about interacting with the enemy's summons. If anything, that would fall under sorcery, but see general guidelines for why I don't think that's a good idea. Instead I think that the sphere used in coordination with summoning should have an effect on the attributes of the summoned creature.
DIVINE should be about interacting with your own units.
Of course like any nice and orderly system, this system must allow for exceptions. Ring of Flames is a great spell even though it breaks this system, and although according to my classifications Bloom and Poisoned Blades should switch positions, it makes more sense to let the priests of the leaves create forests.
LEVEL 2 SPELLS: the thing to keep in mind with these is that they can potentially be spammed. There is nothing stopping a player from building any number of channeling II spellcasters. Thus these spells must not break balance when spammed.
LEVEL 3 SPELLS: these are the archmage/conjuror/highpriest spells. I believe that these should be powerful because a player can only have a limited number of spellcasters with access to them (6, using death mana and liches) and because a significant amount of time must go into researching the tech and leveling the unit. Still I maintain that these spells must not be game breaking, as in it should not be possible to win a game relying solely on level 3 spells (I still believe the current implementation of Tsunami violates this rule; used on a coastal capital it can very quickly end a game).
Finally, I will put in brown below my spells what I consider optional additions. These might be optional because I suspect they'd be really hard to code or because I'm not sure if they're useful/functional/fun/balanced.
Also, you will notice that most of my suggestions pack a little more punch than what has been suggested so far. I tried to model the usefulness of these spells after the most successful spheres out there--fire, for instance, or enchantment.
Okay, after all this blabla, let's talk about the spell ideas I've had.
Channeling I: Escape is a very nice spell that I can see theoretical use for. I've never used it myself, but I think it works. Town Portal might be a more appropriate name
One thing we may want to think about is to allow the adept to bring one additional unit with himself for each level of spell extension he has. I'm a little worried about this because it might be abusable; just add one adept for each unit in your invading army and never fear sudden assaults on the homeland while you're out invading anymore.
Sorcery II: Teleport. Teleports the mage to any tile within his civilization's cultural borders.
Apparently, Kael was planning something like this for Sorcery III. Frankly, I don't see the point in creating a level 3 spell that no one will take. As long as it only teleports the mage I can't imagine how this spell would be abusable. It will make for formidable defenses but only in combination with another sphere. Mages with only Dimensional II will be useless.
OPTIONALLY we could add a fail chance. This could decrease with the mage's level. If the spell fails, the mage may end up anywhere on the map at all. Of course he could try again next round, if he lives to see next round...
Sorcery III: Call Out. Valid targets are both enemy and friendly troops within 3 tiles (+1 per spell extension) of the mage. The main use of this spell is to pull enemy units out of stacks or even out of cities. The targeted unit will appear in the tile behind the caster (from its old point of view). If that tile is occupied, we find another tile around the caster that would be a valid tile to drop the target unit. If there are no such tiles, the spell fails. I can see great uses for this spell: is the enemy defending a city with a City Garrison III Heavy Crossbowman? Pull him out of the city and let your War Elephants stamp him into the ground. Also there could be less obvious uses for the spell. For instance, say an important unit of yours is badly injured and cannot move far away enough from the enemy to escape, but you do have that archmage standing a few tiles from that unit. Cast Call Out on your own unit to get it out of the enemy's range. Also, can you imagine how much fun it will be to create little death traps for Loki with the help of this spell?
I don't know if this spell should be resistible. I'd say no, because it's not a directly deadly spell, but the team will know better.
OPTIONALLY there could be a fail chance here as well, or maybe this will be triggered when the target unit resists. Instead of pulling the enemy unit towards himself, the archmage has fallen victim to Newton's Third Law of Motion: he has pulled himself to the tile one behind the target unit.
Divine II: Dimensional Door. I've been thinking long and hard about teleporting OTHER units. Clearly there is great potential for abuse in this. Still I wanted a teleport other spell at Channeling II, and I think I've come up with a way that should be hard to abuse.
This spell requires TWO spellcasters. One opens a dimensional door at the start point, and the other opens a dimensional door at the end point. One unit can then step into any dimensional door currently open and come out of any other dimensional door currently open. Doing so closes both doors. Like this you CAN bring troops to the front, but only at a rate of one unit per two priests per round. This additional cost (in upkeep and time and resources) should limit the spell sufficiently to keep it in balance.
OPTIONALLY there could be a VERY low chance that the priest opening the dimensional door accidentally opens a portal into an irate demon's bathroom. And the demon was just taking a shower. Instead of creating a usable dimensional door, the spell would summon a strength 4-5 demon in a tile next to the priest.
Divine III: Mass Teleport. Yeah, you read right: mass teleport. This will probably still need some tweaking, but here's my suggestion: a high priest with this spell can teleport all units on the same tile to any destination within his civ's cultural borders, excluding himself. I have a bunch of optional stuff for this spell:
OPTIONALLY, we could offer further target tiles: any tile within three tiles of either another highpriest of the same religion (maybe even one that doesn't belong to the same player?) or within three tiles of any city with a temple to this religion (in this case definitely including cities not owned by the player). If the spell is found too powerful, the highpriest could lose one level when casting it and also lose the spell, so he'd have to level up and take the spell again before he could use it again.
Summoning II: Summon Displacer Beast. Can't believe that's not in the game yet, especially considering this started as an AD&D setting

Summons a strength 3 / movement 2 Displacer Beast. The displacer beast has two special abilities: first, it can displace itself (which is, I am told, not at all like displacing one's keys), meaning it can teleport itself over a short distance (say 3 tiles plus one per spell extension). It's important to note that it can still attack AFTER teleporting itself. I don't know if that's going to be very hard to do with the current system, seeing as mages can't attack after casting. Second, it ignores city defenses when attacking a city (what's a wall to a beast that can simply appear behind the wall?).
Summoning III: Summon Shambler. Summons a horrible extraplanar monster. Strength should be about 10. The Shambler's real power shows only when it's defeated in battle, especially when attacking a large stack. It will randomly do one of the following things:
* Cause horrible amounts of collateral damage
* Infect everyone else in the stack with a disease
* Whither the target unit
* If attacking a city: reduce city defenses
* If attacking a city: reduce population by one (if population higher than one)
* Scatter stack: a number of units in the target stack are randomly teleported to tiles within 3x3 squares of the stack's original position.
I also have a pretty cool idea for a flavor text for the shambler, but let's not get ahead of ourselves
Okay, this should be enough. I'm inviting any kind of feedback: what do you think of these spells?