Since this is an Ideas, Requests and Feedback thread, here's a couple:
(1) Availability of the "Flying" promotion to the Angel race, probably with a fairly high prerequisite like Combat 5. Hey, demons and undead get that Fear type promo available at that prereqlevel, give the Angels something special that helps justify all the winged models. Granted some National Units can fly (Ophanim, Heralds, something else?), but making it open to the rank n' file Angels with oodles of experience adds some new tactical wrinkles. Guess it'd also work out nicely for Brigit, Sphener, or anyone else with the XP to spend on it.
(2) Some more "cast once for one permanent buff" tier III spells, for as many spheres as you can dream up, much in the theme of Death III being not just Wraiths, but Lichdom available. Examples:
(a) Shadow III keeping "Summon Mistform", but also getting an "Invisibility" available to your lucky archmage, basically meaning s/he would get the same promotion as Shadows do. If it's too unbalancing to have a permanently Invisible death dealer like that, a spell granting "Hidden" like Krakens or Ghosts get would be reasonable.
(b) Enchantment III keeping the Create Spellstaff deal, but also a "once per caster" type of generic magic weapon or armor they can create to have carried around by themselves or some other unit, just like a Golden Hammer, Orthus Axe, Netherblade, whatever (but not like the nontransferrable "Master" gear from the various pay-for-promo buildings). Alternately, just have a "Create Wondrous Item" spell that simply pulls a random, non-unique piece of equipment (or consumable like a potion) from a long list, and the Archmage becomes able to cast the spell again if that item is ever lost by your empire (such as it getting drank like a potion or captured in a war). If someone wanted to be really fancy, putting three buttons for "Create Weapon", "Create Armor", "Create Wondrous Item" on when you acquire Enchantment III, and once you blow any one of them off, the other two vanish, well, that might work too, knocking out the element of chance a great deal.
(c) Earth III getting some kind of "Stone Walk" promotion, effectively giving them double speed in hills and the ability to traverse mountains. Tossing around an Earth Elemental is fun and all, but tossing 'em from a mountaintop is better (well, at least until a dwarven druid or one of those flying units noted above comes and bloodies your nose).
(d) Fire III, get some kind of Fire Immunity promo, which hey, means immunity to fire damage, but also the ability to walk through fiery areas like orcs and demons. It's a minor thing, but at least fits the suggested theme that not only might you have water walkers, stone walkers, flying guys, but also guys who can traverse hell terrain etc. very well.
(e) There's probably dozens of threads around the boards with other Tier III ideas, I'm sure there's lots of good and/or better ideas than the above, but c'mon, wizards are supposed to fly, turn invisible, create magic items (other than just from a meteor falling and paying 50gp!), I'm playing to the classics here.
(3) All those "master" equipment promotions, have 'em grant a pittance of gold-loot whenever you kill a unit with them. I'm not talking a huge bounty, even 1 gold per equipment promotion on death would do. So if you have some national unit, especially the recon ones, who can carry around 5 or 6 of these "woods gear", "mantraps", "superior kit", whatever, at least you get a happy little surprise in terms of gold. I would not though, support "dropping" the equipment for anyone to pick up. At best, maybe a small % chance that the unit's killer "steals" any of these relevant promotions if it fits their unit type, as an alternative to getting some gold. So your assassin killing another assassin who has Deadly Nightshade, may have a 5% chance of salvaging it and themselves acquiring the promo. But your knight (mounted line) who kills the Deadly Nightshade user (recon line), would never manage to pick it up... different story though if he fought a Death Knight (also mounted line) with Cataphract Armor. *shrugs*
(4) Religious conversion at the single unit level - I've always hated Recon units being built in an Esus city sometimes missing out on the Esus religion. At the same time though, sometimes you end up with a big Mercurian/Infernals war going on and you MAY feel like micromanaging where your highest XP units are going to end up when they get killed off. How's this for a solution: visit a holy city, pay 1 gold to convert a non-disciple, non-religious hero unit to the relevant religion, casting time, call it one turn. This is regardless of whether the holy shrine has actually been built, i.e., visiting the holy city of Runes, you could convert without worrying about whether the owner (possibly an AI) had built the Tablets. I honestly forget whether the Esus holy city is unknown unless you convert to Esus, but I suppose it could be a subtle form of recon to be in a certain city, and to see a spell available to your unit called "Convert to Esus." This may encourage a little extra Open Borders incentive if you really, really want your recon units to visit the Esus city and need to keep its owner on your good side (until you can stab them in the back and take it for your own).
(5) Bounty Hunter promo payoffs, jack them up based on unit XP, number of promos, or something else that a script / code / whatever makes easy to count. Maybe 1 gold per 5xp? 1 gold per 10xp? 1 gold per 2 promotions? Don't know, the only idea is to make famous/notorious/powerful targets, attractive to a bounty hunter.
Anyhow I know some of these ideas must be repetitive of other ideas "somewhere out there", I've enjoyed playing FF+ so I figured since there's an organized "Ideas" thread, I'd throw in some favourites.