I personally think that being able to create terrain requirements to sigs or secondary sig effects would be interesting. For example, a Sig that can only be done on Grass or Snow tiles (either you're on it or the opponent is, depending), or that gains a bonus when a certain tile is involved (Like a water attack that's stronger on sea tiles, or a non-elemental attack replicating the wind effect by increasing damage on a sand tile).
I don't think that a standardized debuff cost reduction would work (Like an X0 sig point reduction for including the clause) but that some kind of general reduction could work instead, like halving the cost of the clause-attached power. As well, I think that the total of the sig points used should compare to your maximum, as opposed to the actual cost you pay.
For instance. Your sig point maximum is 120. You make a fire attack (on a fire Navi) that deals 80 damage, but it heats metal tiles and deals an extra 40 fire damage (It makes sense since the attack would also turn the metal to Furnace). Instead of paying 120 points for the entire technique, you'd pay only 100, paying half for the metal tile only 40 damage. It would also have a 3 turn cooldown as opposed to a 2 turn cooldown.
For a second example, possibly a little more dangerous, would be a technique that only works a certain tile. Taking that former one as a base, it'd be a technique the deals no damage unless it targets a metal tile. In that case, it would deal 120 damage. It would still be a 3 turn cooldown technique, but would cost only 60 sig points. Halving the price seems like a big deal, but it also makes the attack completely situational. No metal tiles makes it dead weight.
I'm sure many of you can think of ways to positively break this idea to make it completely unfair, but with some small notes I'm sure it could work. The basic reason would be to give some sigs a little more situational flare and allow its form to make a little bit more of a difference.
Aside from that, I like the idea of having more shapes to use like line attack or wide attack.