Basically, each rider has the potential to develop an ability unique to them, which they can even use when not transformed. This technique is therefore not based on their powers--it would be possible for a normal rider to have one elemental attack if it was their unique ability, for instance; it could also manifest as, say, the elemental rider of fire having a spell to douse flames. These are both hypothetical, showing what it can be.
What it isn't, though, are abilities the rider naturally has. For instance, both D.D. and Whitney have shields for temperature active even untransformed as a passive ability, but those are 1: based on their powers, and 2: not unique to them; any elemental rider of their element could (and often would) have those.
So what I mean is, something truly unique to THEM, which no other rider has as a power. These powers are usually based off of their personal interests in some manner. As such, they can vary from completely worthless, serving mundane utilities only, being only effective in certain situations, being very handy, and so on and so forth. Most of these unique powers develop as passives, though: things that take very little energy to maintain, which can have some use but are not really noteworthy in the grander scheme of things.
So, for the main cast, there's a wide variety. Ruby's unique ability is what I previously mentioned: her ability to maintain her transformation. While this is a completely worthless ability for combat, it allows Ruby to maintain female form for a prolonged period of time, essentially allowing her to hang out with friends in her preferred form.
Sally has the most combat-oriented ability: she can turn Tree of Life as a whistle (remember, these are abilities used when not transformed, which is why most don't see use in combat: a rider using them is at human, rather than superhuman, levels, albeit upper level of human) into a lightsaber-by-any-other-name. (Technically, a light-rapier.) For the longest time, this is actually her ONLY cutting weapon.
She has plenty of attacks that can cause blunt force, and her explosions do a lot of damage, and she can even develop specialized rockets for puncturing, but actual cutting is hard to do with a bazooka. Sally realized this, and tried to find a way around it, which is why her ability doesn't match as well to her personality as most: because she realized, during training, that she had no way of using a fundamental attack, and needed to develop one, with this being the only solution she could figure out.
Gary's power is based on how he was there supporting Ruby and Sally even before becoming a Rider. As such, it's an ability activated by his presence near them. Specifically, by touching someone: he basically acts like a magical Gatorade; the person becomes more alert (waking up), replenished, and energetic. However, while this does provide some energy, it's no substitute for proper rest, thus, it serves as a temporary boost to help out when battle fatigue sets in. So, it gives some of what's needed (like Gatorade), but is not a "proper meal" so to speak. Basically, it'll keep a tired fighter able to function, but it won't magically fix them...with the exception of Sally.
Thanks to synergy between her and Gary, both elementally and personality wise, the normally-minimalistic boost suddenly becomes a huge boon, and she's able to function much more effectively. However, while she does get more out of it than everyone else, she still is going to suffer a net negative amount of energy in a fight, slowly being drained even with the support.
Hannah's unique ability is similar to something all spirit totems (and thus, all riders) do. Basically, all spirit totems can act as a one-stop shop for every method of modern-day contact: phone, text, voicemail, email, skype chat, radio, you get the idea. But spirit totems deal with all that type of non-physical communication, things that aren't really tangible. Hannah's ability? Tangible. Her unique ability actually has a name: Infinite Canvas. And it's exactly what it sounds like.
Hannah, essentially, is an infinite source of paper, for writing and drawing. This can be done with it being blank paper, with her writing/drawing with her own two hands, OR, she can send her thoughts into the paper, effectively giving a "perfect" vision of the image or text she was drawing/writing. You know, basically the ability every artist/writer dreams of having. This comes from her nature prior to becoming a magical girl; she was a shy introvert who spent most of her time reading, writing, and drawing.
Vili's unique power is also combat-oriented, but it's less useful than Sally's, because it's largely redundant with what she can do using her electricity powers. Basically, she turns her spirit totem from a knife into a gun, either a pistol or a rifle, with infinite ammo, no recoil, and perfectly-lined sights. This comes from her love of combat, and how she mostly prefers melee attacks: she likes having ranged attacks in her arsenal (especially non-magical ones! Well, non-elemental at least) for variety.
D.D.'s unique power is not directly combat-oriented, though it can help. Essentially, her power is a specific form of stealth: she's 'light of foot', with people unable to sense her through enhanced smell, hearing, or supernatural senses, which allows her to mostly approach unseen: it's not true invisibility, but it's like a dampening field: she becomes harder to spot. This becomes doubly so when applied to a crowd: she can blend into one and disappear, again, not becoming invisible, but even if you know what to look for, having a hard time actually finding her. (Think: the zebra effect.) So while not true invisibility, her being hard to spot allows her to sneak up on people often. The reason she has this power is technically a spoiler, though.
For Whitney, I'm actually thinking of rewriting her stealth mode ability to be this: it didn't really 'fit' with her other powers, yet was a fundamental part of her character. This is a change I'm extra comfortable with, given it would also explain why in that form I envision her as looking almost exactly like her civilian self, and also help to explain why in that form she's not quite professional yet not quite her natural self, either: because it'd be an in-between state.
Now, this does mean her power is combat-oriented and basically a combined version of Vili's and D.D.'s: shooting from a long range, while having a cloak that turns her into a very good sniper, hidden yet deadly. I'm still okay with this, given that Vili and D.D. are younger and less experienced, and that Whitney's a professional. It would make sense for her to have advanced training when developing her unique ability.
So, very happy about this decision overall. Might require a little tweaking here and there of character things, but nothing that I wasn't already heading towards changing anyway.