You can play multiplayer against bots if you don't have any friends to play with. There are three modes to that: survival (be the last one alive), point battle (be the first person to get a certain number of kills), and time battle (Like point battle, but timed instead of going for a certain number of kills), which all share the same four map choices. The bots have adjustable difficulty settings and you can also do stuff like toggle the new items for it on and off.
Beyond that though, this mostly is a score-attack game. The replay value, once you've played through the game at least three times to have visited every planet (and even then, there's still stuff like hidden warps and the like you can find and take to unlock other possible routes), mostly comes from getting the highest score in each planet possible. Getting a medal (obtained by getting a designated number of kills, without letting any of your teammates die) on every planet will unlock an Expert mode, for more difficulty (and gives Fox a pair of sunglasses).
If it's anything like the original, doing stuff like that will also unlock some tidibts in the multiplayer mode, but I haven't completed getting the medals yet, so I can't confirm if that's still the case, nor have I really heard about that from anyone else (based on what the menu looks like to start with though, it looks unlikely, which is kind of disappointing to me, as messing around with them was one of my favorite things about the original multiplayer).
