As others have mentioned, if Frontier wanted they could keep putting out DLCs for PZ for years to come. There’s an abundance of themes and iconic animals that we still don’t have in-game. The issue just comes down to if Frontier feels that the additional time and effort is worth it financially (which I personally feel it is but I’m a little biased as an avid fan of the game).
For future packs, personally I’d like to see some game-changers with new mechanics. So far the only thing we’ve had like this was the Aquatic pack, which I feel has been the best pack we’ve received to date. With this said, my top picks for future packs would be a marine pack and avian pack. We’ve received some great semi-aquatic species, but still lack any true fully aquatic species. Large bodies of water still feel kinda empty without any fish in-game. Same could be said for the treetops and canopies of our zoos, where are all the birds? There’s so many iconic species that could be in these packs; sharks, rays, octopi, sea turtles, manatees, eels, eagles, owls, parrots, hornbills, large bats, the list goes on. Honestly there’s so many iconic species that would fit in these categories there could be multiple packs. For example, a River pack with an arapaima, catfish, alligator snapping turtle, and wading bird would be really unique and better diversify PZ’s animal roster.
In a similar vein, since we more-or-less know what the next pack will be, I’m hoping we may see some new mechanics with it. Three of the four discovered species are known to burrow so I’m hoping we may seem some digging or burrowing mechanics introduced with them.