Naturally, science is not predictable, and the most interesting discoveries between now and 2028 will be the ones we cannot predict! But planned projects already hint at where the science frontiers will be when LISA operates:
For example, massive progress can be expected in transient astronomy. Telescopes like the LSST and the Square Kilometre Array (SKA) will likely identify new systems that flare up irregularly or only once, and there is a good chance that some of these will be associated with gravitational wave signals.
As another example, extremely large telescopes (EELT, TMT, GMT) and large space telescopes (JWST) will be observing (proto-)galaxies at unprecedentedly high redshifts, at which LISA will simultaneously observe individual merging black hole systems.
The expected progress in all kinds of electromagnetic astronomy will sharpen the need for complementary gravitational wave observations of the unseen universe, as well as providing a wealth of information that will make it easier to identify the gravitational wave sources electromagnetically. LISA will pioneer gravitational wave observations in the rich frequency band around 1 mHz.
In summary, by 2028 our understanding of the universe will have been dramatically improved by advanced observatories of electromagnetic radiation. Launching a millihertz gravitational wave observatory will add a new sense to our perception of the universe and enable us to listen to a world that is invisible with light.
LISA will be the first-ever mission to survey the entire universe with gravitational waves. It will investigate the history of the universe back to redshifts of order 20, test gravity in the dynamical strong field regime, and probe the very early universe at a time when its energy scale was in the important TeV range. It will serve a unique role in the scientific landscape of 2028.