When i got this as a gift 6 years ago for my Xbox, it quickly became one of my VERY favorite games.
It does not end there. There is a system in the game called "Mapmaker" that lets you create your very own map for playing in these modes. It can be altered within the map, so you can make it a Story Mode map, create objectives with the trigger set, and also set bots, weapons, keys, crates, power-ups, and other items to help along the way.
Characters? Of course, there are HUNDREDS of characters to choose from. But the only drawback about them, is you have to unlock them, thus adding a challenge.
Speaking of challenge, if you're searching for a challenge, Timesplitters 2 has a challenge mode. these challenges range from throwing bricks through windows, to infiltrating the Siberian Dam (you must not be seen, you will fail the challenge. Plus, you are timed, making it harder.)
So, Timesplitters 2 is the next Halo. (I think Timesplitters 2 is better.)