Mass agricultural slavery would definitely be phased out (economically unnecessary by the 1920s-1930s at latest due to the end of the need for that many people working in agriculture) and that that would probably lead to a mandatory deportation to Africa of a bunch of no longer relevant slaves.
Where would they go? I doubt European colonial powers would be interested in accepting ex-slaves from the US, importing westernized blacks to Africa seems like asking for trouble.
Not sure whether the Americo-Liberian elite would feel threatened by an influx of poor Black Americans, but it seems like it would undermine the prestige of being Americo-Liberian and the perception that they were superior to the natives which they relied on, even if it would boost their numbers, and the new Americans would want their share of the spoils. They did take a few Afro-Caribbean immigrants until around 1930, but that was much smaller numbers and often people with some education and funds, mass immigration of dirt poor ex-slaves (mostly rural laborers) would have been very different.
