What features of grasses would need to exist in a plant for the plant to fill the same niche as grasses in a grassland?
Rapid growth and spreading.
Willing to grow in marginal and poor soils.
Ability to survive grazing, including really severe grazing.
Ability to survive being stepped on by absolutely everything in sight.
Strong root system for stabilizing soil.
Ability to either survive fire, or rapidly reseed and regrow after a fire.
If in a region that experiences frost and snow, it should be able to at least regrow rapidly after a frost, better survive and thrive even after being frozen solid.
Capable of letting the upper surface "die", to provide reduced water needs and shade/thermal protection in event of drought/water shortage.
Ideally, should be significantly nutritious to eat, as this invites grazers to trim old growth and fertilize and spread seeds.
Should not grow tall enough for a 'grass' fire to become a 'forest' fire.