Pikachu is the first thing that comes to the mind of many people whondon't involve with pokemon. It is because Pikachu is the mascot of the Pokemon franchise.
What pokemon would you chose to be the mascot of pokemon? Well, i'm happy with pikachu, but if i'd be given the opportunity, i'd chose Riolu. I don't have any specific reasons, but you could! What would your pokemon mascot be?
I think it's about 20 years too late for a new mascot, but if the Pikachu family somehow disappeared into the Distortion World or something, there wouldn't be a whole lot of Pokemon to choose from. If you're going by just a marketing standpoint, I have to think that Charizard would be a frontrunner, along with Eevee.
I think it would be fun to have dual mascots though- Mew and Mewtwo. Gives the franchise a combo of cute and cool Pokemon that should be recognized by even the most casual of fans.
Azumarill: evolution of a Pikaclone which shares many cute-y traits with Pikachu, and being the final stage there's no evolution to be overshadowed like Raichu is. Marill also used to be featured in merchandising years ago so it's not that obscure.