Here's some things I think would help Rush Bros. and are all things I would add if I was offered the opportunity to change things up.

Multiplayer needs a big fix. The first thing I would do is create a leaderboard. This leaderboard would not only allow you to see another player's times, but download the ghost and race against it. This opens up a larger range of players, because anyone who's played that race/mode will have a ghost you can race against. Whenever you beat your own score, it removes your old ghost from the list and replaces it with a new one. When someone who has downloaded your old ghost connects to the server, it will ask them if they want to update your ghost file or delete it.

Using that feature, when a player is attempting to find a game with specific features and is unsuccessful, it will automatically download a ghost randomly using their leaderboard ranking as a base and choosing a ghost from within 2 seconds of the player's high score.

The menu system needs an overhaul, it's too difficult to navigate. Many of the game mode features are buried under submenu's and need to be reworded for clarity. It is difficult to find the single-player game modes at all.

An overall rank for matchmaking needs to be implemented so new players don't get stuck going up against more experienced players all the time. Maybe also takes into account your unfinished races. People who tend to stay until the race is finished need to be rewarded by being grouped up with each other more often.

Split-screen needs to have at least the option for the characters to show up on each other's screens, just like in multiplayer.

Multiplayer matchmaking should work like this:
When players are matched up the player who's been waiting longer gets to suggest a track, Player 2 then has the chance to accept or choose a different track, if Player 1 doesn't accept Player 2's track, then the server picks one they have a similar leaderboard ranking with, and plays it. Then after each race you can vote to Play Again, Play Next, Play Random, or Choose Track. If you lose the race, the winners Ghost is downloaded for that track so you can race it again some time in single player.

It desperately needs the ability to capture AnyJoystickButton || AnyJoystickAxis and enable the joystick for player 1 automatically. Considering split screen is a seperate mode already, it should be easy to flag the check in single player mode, and then maybe offer a prompt in split screen mode to choose player 1 or player 2.

A new Tournament Server mode, allowing more than 2 players to race each other in a ladder-match style tournament. Losing racers can view the other races, and are the ones who vote for the tracks the winning players will face off in.

Rush Sister's and Zombies was not what this game needed. It's based on multiplayer, and it needs a better multiplayer experience all around.