I am building a world where the flora and fauna is based on that of Gondwana, the lost supercontinent that consisted of South America, Africa, Madagascar, India, Australia and Antarctica. One of the key biomes of this continent, linking all the other regions together, is a vast savanna stretching across the continent in the tropical and sub-tropical regions.
The wildlife of this continent is based on those creatures that evolved after the fall of the dinosaurs, but only on the southern continents. Many of the most common land mammals that we know of today evolved in the north (Laurasia, at the time) so the only mammals on the continent are prototheres (egg-layers), marsupials (opposum and kangaroo and more), xenarths (sloths and anteaters), and afrotheres (elephants, elephant shrews, hyrax, and aardvark).
If you look at those animals, you will see preponderance of insect eaters, such as aardvark, armadillo, anteaters, echidna and more. So, this sort of mammal ecosystem is best justified by a dominance of termites as the primary herbivores.
What changes, if any, do I need to make to a world to justify termites being the primary herbivores on a semi-open savanna?
The competition from termites must be such that grazing herbivores never evolve. In turn, mammalian carnivores that specialize in grazing herbivores (wolves, lions, etc) never evolve. There are a couple of shrub browsers (kangaroo) and a few tree-destroyers (elephant and giant sloth), but nothing like wildebeest, horses, or buffalo.
The savanna biome is immense, covering nearly half the continent, an area larger than Africa. There is a mix of grassland, open forest, and closed thorn thicket. The termites must be dominant in all parts of the savanna.
If climate affects competition between termites and herbivores, it can be adjusted to favor the termites, so long as the mix of tree and shrub types is possible.