First of try looking at BHCreations, that really helps in developing ideas. Try typing words into a Thesaurus or Rhyming Dictionary and seeing the results that coming out. As for gimmicks, it seems like the most thought of one here is a Dual Element theme, where each Skylander has two elements. Perhaps you could enhance a certain aspect that you like, for example speed.
You could try something like, you can put one magic item on the portal and it doubles a certain stat, for example strength, health or speed. The name I had in mind was Skylanders: Rune Force and the magic items would be called Runes.
That's only a suggestion so fell free to add onto that one. Also keep in mind that most users here won't give away good ideas willingly because they can be hard to come up with sometimes, especially one that could top the latest instalment.
That doesn't specify that it necessarily doubles a stat. Also, Relics are set to a certain Relic Master (Half of them anyway). These ones (if this user keeps them that way) are set to a certain stat, for example for have Strength Runes, Health Runes, Speed Runes etc.
Also, keep in mind, if a user makes a suggestion, then they are entitiled to use that suggestion for their own purposes if they want to. The suggestion is still yours to use as you have full permission to use it and make adjustments to the suggestion but the person who the suggestion origination from can still use it.