I sincerely believe that the upgrade paths are going to be limited to Focus A + Focus B + Focus A -OR- Focus A + Focus A + Focus A. Every other combination, e.g. Focus A + Focus A + Focus B -OR- Focus A + Focus B + Focus B has resulted in nothing. To me this is a clear indicator that other such combinations will result in nothing.
Also, I mentioned it before but there is a limit to the number of uber effects that the Devs can put on items, going the route listed above there is a result of 36 possibilities which is already A LOT. And, I'll throw this out there again since it hasn't been disproven yet...the Devs DID mention a "special" upgrade route that was not related to Focus type which I believe may end up being based on Gem Type.
Edit: Here is a list of what I believe are the other possible options for upgrading based on gem type:
Focus Type | Essence Type | Gem Type 1 | Gem Type 2 | Gem Type 3
Air | Ethereal | Dominion | Escalation | Opposition
Earth | Ethereal | Dominion | Escalation | Opposition
Fire | Ethereal | Dominion | Escalation | Opposition
Negative | Ethereal | Dominion | Escalation | Opposition
Positive | Ethereal | Dominion | Escalation | Opposition
Water | Ethereal | Dominion | Escalation | Opposition
Air | Material | Dominion | Escalation | Opposition
Earth | Material | Dominion | Escalation | Opposition
Fire | Material | Dominion | Escalation | Opposition
Negative | Material | Dominion | Escalation | Opposition
Positive | Material | Dominion | Escalation | Opposition
Water | Material | Dominion | Escalation | Opposition