It probably mostly depends on whether or not she wants a convention speaking role. Romney, McCain, W., etc. didn't speak at the 2016 GOP Convention not because they weren't invited to, but because they decided to not go. Usually, ex-nominees and ex-presidents are shoo-ins, with pretty much the sole exception of Richard Nixon.
Romney, the Bushes, etc. not speaking at the RNC in 2016 is an unusual circumstance: They refused to attend the convention because they didn't like the person being nominated: They wouldn't endorse Trump in the general election. Barring some really unexpected scenario though, that won't apply to Clinton in 2020. She'll (presumably) endorse the nominee. If she skips out, it'll be more like Bush skipping the convention in 2012: Potentially a move to not create some kind of "controversy" over a polarizing figure appearing at the convention. That assumes of course that her favorability #s will still be poor in 2020. If her #s have recovered by then, then of course she'll speak in primetime.