While this may change sometime in the future, as far as we know, there are no new heists on the way for GTA Online. New gameplay mechanics are introduced in updates like Further Adventures in Finance and Felony, but for some reason, the single best received and best GTA Online DLC isn't getting revisited.

The Heists update was only released in 2015 in March, in spite of the eponymous heists being kinda the whole point of GTA Online and the main endgame attraction. In spite of there being countless methods to make money in the game, Heists are still the most popular simply due to the massive payouts. While VIP work and CEO work both have an equal if not better money-to-time ratio since the Pacific Standard heist takes upwards of an hour to complete, these options are not as popular.
Which is what brings us to the reason: Rockstar is only giving us alternatives to Heisting which are in the same league as heisting, because given a method to acquire money that is more efficient than heisting, heisting becomes obsolete. However, if new heists are released, then they have to be more potent than the current ones, or the community will be disappointed.