He might try to unify them all into a Neo-United Provence's of Central America. Perhaps trying to Revive the Republic of Yucatan and break the control of Great Britain on the Miskito Coat and British Honduras. He might also lead an expedition Aginst costa Rica and Panama
If Walker attempts to expand, I can't see how he can survive. There is too much against him. If he behaves and doesn't scare his neighbors, he could survive. Anything that provokes an intervention probably overthrows him, and that might include any re-institution of slavery.
To be honest, I don't see much history changing. We have one of two options. The first is that any US settlers mix with the natives and becoming part of the mestizo elite, or the US settlers stay racially apart. The first leads to almost the same history as our timeline. The second may see improved government in the short term (assuming the leaders adopt best practices of the US government instead of pillaging the place as a corrupt, unaccountable elite) as trained US citizens run the government, but who have little long term viability. In the second scenario, if Walker and his descendants built up the country, then Nicaragua has a chance of becoming more like Costa Rica before the natives overthrow them by the 1920s and adopt native rulers. If the pillaging occurs, they revolt comes sooner and again we're at our timeline.
The only major change is that if the Walker regime is long term (several decades) and beneficial (both unlikely, but possible), then Nicaragua might become the preferred place for a canal instead of Panama. At that point, prospects for Nicaragua look up and Panama remains part of Colombia.
Nothing, however, substantially improves the situation above what is the norm for Central America. At best we're looking at a Costa Rica situation - something good for the neighborhood, but not amazing. At worst, it simply makes Nicaraguan politics even worse.