It's not unrealistic to think that the next Sonic game will have new visuals. Smart phone games are usually meant to be low budget, so it makes sense that they are reusing lost world's assets for this game. Also, as cool as smartphone games with cutting edge graphics are, they cut out a large portion of a potential audience due to lots of people being on older phones. Lost World's graphics work perfect for a cell phone game. Game honestly looks like it could be fun, too. Unlike Lost World.
It will be a bit depressing if this is the major Sonic game for 2015, but I suppose that will make 2016 all the more special.