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.
However, releasing a heist more lucrative than Pacific Standard won't happen simply because it isn't worth it for Rockstar. PacStan already yields a good million if you get a generous cut on hard without losing cash from the money-bags. If players are given a reliable way of grinding more significantly more than a million per heist, Shark Cards will immediately look a lot less appealing.
Shark Cards are what are keeping GTA Online supported with content. Considering how popular they are, having made as much money as they have, it is no surprise that they are the main source of the budgets for all the free DLC GTA Online is getting.
Many people openly oppose the use and promotion of Shark Cards, however Rockstar's system of microtransactions is perfectly moral, especially in light of some other games out there. Currency is the only thing you can buy with real cash, and there are ways (covered in our guide) to acquire said currency with regular play.
There are no overpowered items or vehicles that you can only buy with real money. There are no timed buffs such as decreased damage received or increased damage dealt that you can buy with real money. This mentality puts Rockstar well above many other developers of (mainly free to play) online games.
The truth is that DLC takes resources and work-hours to make. Ergo, money. Since you're not required to pay a monthly subscription for GTA Online, that money comes from Shark Cards. These cards are very fairly priced and the revere-sale promotions are so frequent these days that those amounts are pretty much the standard now. However, a new, lucrative heist would decrease Card sales, which would either result in fewer DLC updates, or updates with less content.
Have you bought any Shark Cards in GTA Online?