Pretty sure this kind of thread has been done before... *shrugs* oh well...
I'm not one to make guesses... I like working with evidence... so this is probably more wishful thinking than a prediction, but... I'm gonna say the next DLC will revolve around Claire Dearing and the two sides of her character; the well-being of the park and the well-being of the animals.
I'm thinking guests and their needs will be more fleshed out (people will have names, needs and things they are interested in seeing), smaller things like bathrooms, small food and souvenir stands, and useful objects like benches, streetlights, vending machines and trashcans will be added, as well as some basic decor items like flowerbeds, fountains, statues and decorative fences, the kind that keep guests contained... animals? Not so much. A few new guest facilities (like maybe a jewelry store? Or a taller hotel to house more guests in the same amount of space?), at least one new attractions (perhaps the classic Jurassic Park tour ride? Or the savannah tour seen in Jurassic World?), and new management staff (maintenance workers, to keep the park clean and repair damaged benches, trash cans and such when they're inevitably destroyed) would round out the park side.
For the animals, the current system for adding and removing trees and low brush will introduce "themes" featuring different forms of plant life (and ground-cover) to replicate different biomes. In addition to the one and only Jungle theme currently available, I'm thinking a Temperate Rainforest theme, either a Semi-Desert environment or a Wetland theme... and a generic suburban theme with simple trees, grass and flowering plants for guest areas. Individual trees (and rocks) will be added, although I'm thinking these will be more decorative than practical (especially the rocks) since the dino AI will likely still focus on the plants added via the biome system rather than these individual objects. New dinosaurs will be added, of course... no new hybrids (not Claire's area), but I'm thinking the new dinosaurs will be both in-demand and very exotic, perhaps Therizinosaurus, Pachyrhinosaurus, Plateosaurus... probably two or three more, but these seem the most likely to me... the wishful thinker in me wants to say Dimetrodon too.
Lastly new story missions and maps to go with them will be added, including a new (and bigger) sandbox mode map.