Spheres were only added as Kael came up with spells for them. I remember a time when the Air sphere did not exist--or rather when only Air III existed and only Hippus UUs had access to it.
A while back Kael said that they would be added if people suggested good, interesting, and balanced enough spells to fill out the full 3 levels of each. He does not want one sphere to outshine the others, and considers most of the interesting suggestions he's heard to be overpowered.
That was however before FfH2 officially went into feature lock, so I think their appearance in the main mod is unlikely.
Kael said that if he did implement them that Dimensional I would again definitely be Escape. He also said that for Dimensional III he would like a spell that opens up a portal (like those used in the The Cult scenario to link the dungeon portion of the map to either side of the mountain range), but that he could not come up with an adequate way to let the player choose the location of the portal exit and that it would probably be impossible to teach the AI to understand how to use these shortcuts.
In my version I have a working way of targeting the portal exit. I make the Dimensional III units summon a "rift," which acts like an ICBM and can thus move directly to any tile on the map (provided it is not too close to a neutral civ's units or territory). I then gave the unit a spell whose effect is actually handled by the python prerequisite. This effect identifies the Rift's tile and the tile of its summoner, and if they are not the same and neither of them have a City, Portal, or permanent improvement it places a Portal there and connects the two locations.
I had to immobilize the summoner in order to prevent it from making a portal when and if it moves to a nearby tile. The python prereq for the Rift's Open Portal spell thus also undoes this immobilization. Also, after many attempts last night I realized that the Rift must be domain_land instead of domain_air or else it will just create fallout and not actually move to the targeted tile in order to make the portal. Since it is considered a nuke, the process causes the map to shake, claims that the unit exploded, and causes diplomatic penalties. The text claiming the explosion can easily me made blank by editing CIV4GameText_FFH2. Removing the diplomatic penalties would require editing the DLL, but editing CIV4GameText_FFH2 can easily change it from "you nuked us!" or "you nuked one of our friends" to something like "you've used magic that threatens the very fabric of the universe!" I personally think that the diplomatic penalty from such powerful evil magic would be a very good thing if it could be made so that some (Infernal and Sheaim) leaders reacted positively instead of negatively to it. I believe that would require only minor changes to the SDK.
I have an alternate, more AI-friendly Dimensional III spell too. Rift Gate creates a requires-caster building of the same name, which works just like an obsidian gate (and the number of units you can airlift stacks so it is not made obsolete by the normal building) but has a risk of causing a "nuclear meltdown."
I've toyed around with a lot of different Dimensional II spells in my versions, but generally not been very happy with them. Most of my ideas required abilities beyond those of unmodded FfH2 but which might be possible in an FF based modmod. My current Dimensional II spell is simpler though, and possibly better. Abjure is not actually cast by the unit with Dimensional II, but by summons in the same stack (leaving the caster free to cast other spells that turn). The spell allows limited-duration summons to forsake Erebus early and return to their home plains, in exchange for giving their xp to the caster who let them leave early.
I also make Dimensional I, II, and III respectively grant Combat I, II, and III as summon perks.
I always make Force I be Temperance, which grants a promotion of the same name to the stack. This promotion grants resistance to both holy and unholy magic.
Normally for Force II I do something much more complicated in python, like repelling priests, angels, and demons and doing a little damage to them. Right now though in my version I took the lazy approach of making this the way to gain the Magic Resistance promotion.
For Force III I have Summon Runewyn. In keeping with the Bestiary, Runewyns are flying angels that are Magic immune, can target arcane units (and Archmages in particular) in stacks of stronger units, and even when they loose have a chance of removing spell sphere and channeling promotions from the stacks they attack.
My current Creation spells are not all that great (as in interesting; they are probably overpowered) right now as I've mostly been limiting myself to xml (actually excel) editing recently and I somehow broke the old python based spells. Creation I is fertility, which creates a building that gives food and health. Creation II right now is Baby Boom, which increases the population by I. Right now Creation III is summon great person, which for now just gives a free shade.
If I can get my old Prodigious Birth to work again Creation III may cause the birth of a random great person in a random civ's random city. I've also considered a spell (perhaps the second for that promotion, as in m version many tier 3 spell sphere allow 2 spells) that increases a tile's food production directly in python, but often fails. I Might also make one that has a chance of creating a random resource on the tile. If I come up with a better Mind 1 spell than I'll go back to having Creation II be Inspiration, which should allow free specialists of all kinds and provide some GPP and culture in addition to research.
I guess I'll go ahead and mention that I've managed to keep my version stable for longer than I'd been able recently (due largely to relying on Kael's excel based editor rather than editing the xml directly and possibly introducing typos and syntax errors there), and am considering releasing something soon. It won't be as ambitious a project as I'd like to release eventually (no custom art, schema, dll, or scenarios), but it seems like a good idea to release something before I break it or before I have too much school work for modding again.