Well, there are always a few new monsters in each AP volume, and there is the chance an upcoming Campaign Setting book has some. So there is a slim chance overall.
Personally I find elementals rather dull creatures - each type has only one or two interesting things, no matter the CR, so more types wouldn't add much (in my opinion). This has an advantage, though: It's relatively easy to rework an elemental type into another. Water elementals without their special powers but with an ability score damaging touch could make good void elementals, while earth elementals are a good base for wood elementals etc..
If you find the 3rd party elementals balanced - maybe make an exception from "Paizo only" and include them into your campaign.
You don't want your players summoning time elementals. There are plenty of third party elementals, but they were designed as monsters, not as the subject of summons spells. It seems unlikely that there will be more created at this time. Paizo did develop the lightning, mud, ice elementals and the like. That's probably as far as they are planning to go.