Frozenbyte is definitely going to work on a new Trine game in the future. "As for the Trine franchise, it is clearly our most successful," a rep for the developer commented, "making a third installment a no-brainer."
However, the developer doesn't want to rush into a Trine 3, because "we don't want it to feel like Trine 2.5, but rather a large leap and something even more awe-inspiring. So that might take a while."
Perhaps in the interim, the developer will explore other IPs, as they note that "this is the best way for us to grow as a company." The developer is also mulling over the possibility of additional DLC for Trine 2. "We would really like to bring more DLC and maybe the Magic Mayhem later on, but currently we are contemplating if it would be better to put in another long development cycle to get out high quality DLC or if we should just turn our focus over to a third Trine game," the rep told GamingBolt (via Polygon).
Given the year-long development on the Goblin Menance DLC, it seems unlikely Trine 2 will see many more expansions. However, "2013 will be quite a year for us and we aim to please with some cool announcements," the rep teased.