Ok, then I'll make a ghostly race that I'll name Nightmare Wraiths
So you'll probably have Destroyer Form and Mount as core functional elements. Maybe shenanigans with Possession-based abilities and triggers.
I post again because I forgot a few transformation mechanics, and more summoning mechanics are also usable.
Druid Swap: The Druid of the Claw and Druid of the Talon are both units that permanently change forms through an ability, one that, like some other units, uses the alternate form model tag and transformation animation. Unlike many other methods, the amount of buggyness is nearly nonexistent, as it's purely "turn into X". This is because Blizzard has alternate forms
always be classified as separate units, instead of a shift to the base unit, meaning a lot of things they do with models aren't
required for functionality in any meaningful way.
Inferno Artillery: The Inferno spell is the only AoE that summons automatically, making it useful for units like Starcraft 2: LotV's Ravagers that perform artillery functions by casting an ability. This is particularly useful when the summoned unit capitalizes on the stun in some way, either via AoE damage of it's own or an ability to extend the crowd control effect, possibly by making units to bodyblock.
Arrow Reanimation: Black Arrow, essentially, makes it so that kills will
always create a particular unit, provided the unit dies within a specified time limit. This means that one can make kills add units of a variety otherwise unavailable. The time limit means that kills by unconnected units, such as Creep fatalities, don't generate the unit, and also ensures that attribution properly occurs. For area of effect attacks applying such an effect, a basic trigger can replicate the effect with basically anything.
Doom Creation: The Doom ability is interesting. It's the only ability that has a permanent duration dealing damage. This means that it can be used to mark a unit for inevitable death. Usually, it gets used to create a major unit that'll wreck enemies further, rather than being primarily a dedicated source of damage or making a swarm of units. Guaranteeing the death of a unit is worth quite a bit, which is why such effects are usually restricted to Hero ultimates.
Factory Stream: The Pocket Factory's ability to produce units is tied to the ability that spawns it. This is, itself, fairly useful, as it lets you have an ability to summon a unit with an extra trick it would not have by default (potentially useful for saving on data and models). However, an unused ability exists to do this as a passive, so you could apply it to units and structures to produce a technically-not-trained unit at a constant rate, on a timed life. Such as a unit used as a resource for abilities, or the "foundation" unit a lot of others interact with for additional forms.
Engineering Loadouts: Activateable Engineering Upgrade abilities can allow one to alter a unit quite considerably by swapping abilities without
technically being a different unit, preserving a few behaviors, such as Mount/Pick Up Archer qualifiers, at the cost of needing to navigate some
spectacularly weird things. Generally, switching from a summoner to a damage dealer is doable with fairly little difficulties, as a number of mechanisms for damage dealing also have summon versions. And in the worst case, all target-point abilities are equally able to be cast through Rain of Fire.
Stone Regeneration: Funnily enough, the Stone Form of Gargoyles actually specifically mentions the additional 8 HP/sec granted as part of the ability, rather than the unit. This means that a unit gotten from the Stone Form ability can have additional health regeneration over other access methods. Much like the Druid forms, there's no bugginess to be had from not having the ability on the alternate form, nor does lacking the ability on the normal form cause particularly nasty issues.
SEF Fragmentation: Storm, Earth and Fire is actually a unique ability. Not only is it the only morph ability that morphs into multiple other units, but it's through a list, so you can just add a unit to the list multiple times to get it more than once, unlike a massive number of other abilities that are restricted to just one or two unit types. This is primarily useful for large chains of merges, upgrades, morphs and revivals to break back down temporarily, though the ability to make units for those could prove useful.
Parasite Poison: The duration limit is, itself, fairly important to re-defining the dynamic. Doom's permanent duration allows one to guarantee the loss of something,
eventually (bar debuff removal), while Parasite acts as a conventional DoT that also makes a minion if the enemy dies during it's duration. This means that much weaker units can have Parasite-based abilities, while Doom has to be on higher-tier units as a general rule.
Spawn Splitting: The passives to generate units on death can be used to accomplish many things, and in this case, what's important is that the death makes a
different unit, which can cover the weaknesses of the unit that just died. For example, a unit akin to a Mountain Giant, big single-target beatstick, could make units that offer area of effect damage that can kill off the sort of meatwalls that typically cause issues for single-target beatsticks.
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A further note on transformation abilities: They do, in fact, have the ability to make any of them act like Destroyer Form (matter of fact, Destroyer Form has
all the same fields as Gargoyle Form), as the payment option is a morphing flag, as is whether or not it's permanent. I'm not particularly in the mood to mess around with the "needs payment" at the moment, but it
may be usable to have an "immortality protocol" that causes you to lose resources when the unit triggers the revival. A useful balancing factor, in some cases.
Edit (Forgot Lava Spawn):
Lava Cloning: The Summon Lava Spawn is the last one of these, I promise. It's the one creation ability I missed (that I know of and isn't redundant), and it's a pretty crazy one, because it defines how many attacks before a split and a duration the split takes. And a generation limit. At any rate, it allows one to make a summoning spell that has the summoned unit get the ability to clone itself by attacking. Also, it uses the Alternate Form animation set, so that's a modelling concern (Druid of the Talon turns into a bird for the delay, for example)