I think the final shot was about cementing two of the things the movie tried to do: the power of a symbol and the idea that the force can be powerful in anyone no matter how humble their origins are. It's an explicit rejection of the fanboy nonsense about working out how everyone is potentially tied to the original cast or trilogies in some way. It also touched upon the lesson that Finn was supposed to learn, that he can't be selfish and has to consider other people if he wants to be a good person and do good.