I can see two interconnected possibilities:
1) a larger population base of Americo-Liberians. This means probably more investment into the whole colonization scheme, although the OP requirements make it difficult because it would be more effectively done before 1847. This would also probably cause a larger part of the coast to be part of the country.
2) a serious commitment to build a free and inclusive society. I don't think it'd be easy or straightforward, but I think that the Americo-Liberians had a chance at creating a situation where the native African population is included into the civic body, as opposed to being marginalized and oppressed. I still expect the elite being mostly composed of immigrants and their discendents, especially in the first decades.
That should offer Liberia a basis for further development.