Terraria does not have an end. It's a sandbox. Feel free to do whatever you want! You are using a Muramasa, but it's not the strongest weapon, it's actually one of four items needed to craft the strongest one! (Night's Edge)
Go build something great! There are many forums where you can find people building really amazing things in Terraria, like castles, big forts, houses, etc.
Pixel art! Yes, Terraria is just block, blocks and blocks. Various colours, textures - this can serve as a great set of "painting" tools. And what's the canvas? The world!
PvP. Like it was mentioned in other answers, you can fight other players.
Download custom maps - there are tons of custom maps already, with maze games, adventures, role plays.. Just pick one.
Last but not least - wait for the 1.1 update! It's gonna be huge ;)
The release of 1.1 is planned for the first of December, and there are going to be really interesting things!
NPC names are saved as text in the world file. I'm not aware of any save editor that can rename NPCs at this point.
However: There are currently* 35 possible names for the Guide, and — one of them is, in fact, Brian. Since the name is completely random each time, there's no telling how many of the gentle Guides you'll have to mercilessly eviscerate before finally getting a Brian.
Although, at about the 24th slain Guide, your chances of having gotten at least one Brian will approach 50%.
Hex editing
If you really want to rename your NPCs badly, you can do so by editing your raw save file with a hex editor like HxD. This should go without saying, but make a backup first because it's very easy to fry your save file.
Currently*, the NPC names are at the very end of the save file. They're saved as strings with length prefixes, so edit the name to be whatever you want, then edit the byte before it to reflect the length. For example, "Brian" has 5 characters, so you'd edit the preceding character to be \x05 (ENQ).
The maximum length for NPC names is 22, and obviously they will reset if the NPC dies.
* Terraria 1.1
Best Answer
You need to wait about one in game day, or simply exit and re-open the world.