I do not know what Aether's connecting theme is. It has been associated with Improvement, the Divine, dimensional travel but none of these seems to connect IMO. Improvement doesn't explain Lightning, The Divine is broad and undefined and dimensional travel seems like an aspect instead of a whole theme.
or
Although Air and Time have been dissected for personality (freedom vs velocity of time) they have used this personality in limited ways. Time's speed up version is restricted to draw power right now. These limited options restrict brainstorming much in the same way that ignorance of another floor limits one's exploration.
Earth lacks much creature definition other than "above the curve" as such it is hard to determine what an
skill is. It also overlaps thematically with Gravity a lot but that is a minor concern.
Light is a very self orientated play style in contrast to Darkness. Most effects in the game are interactive. A
is probably the easiest duo card to come up with of the
duo card possibilities.
So a Duo card using any of
would have the most problems.
Of course opposing themes (Life's growth and Death's decay) are more difficult than perpendicular themes however even those are possible (Infection a growing source of decay)