Should you start a new game for the Elden Ring DLC?
I recommend not starting a new game for Elden Ring's DLC and instead taking your existing character into Shadow of the Erdtree. This is primarily because the expansion is balanced around the endgame and having an endgame-level build. It's arguably worth doing if your existing character is in a New Game+ cycle, though.
Why you shouldn't start over for the DLC
While some expansions warrant starting a new game or character, these are quite literally game-changers that overhaul base game systems and mechanics in several meaningful ways; think Cyberpunk 2077: Phantom Liberty. The Elden Ring: Shadow of the Erdtree DLC, though, is not that kind of a shake-up, and instead is entirely centered around the ridiculous amount of new content it's adding (the playthrough I did for my Elden Ring DLC review lasted 36 hours, and I didn't even find everything).
This is reinforced by the fact that accessing the DLC requires you to kill two bosses — Starscourge Radahn and Mohg, Lord of Blood — that require significant mid-game and endgame progress, respectively. Getting to and then defeating both would take quite a long time to do with a new character, and while it's true that elite speedrunners can beat every major boss in under two hours, mere mortals like you and I aren't that skilled. I strongly recommend not starting a new game for Shadow of the Erdtree for this reason alone, though there are a few other good ones to consider, too.
One is that the DLC is balanced around having an endgame-level build, as you have to kill an endgame boss to enter it. Even if you managed to somehow quickly take out Radahn and Mohg while underleveled, you'd have to then go into all the brand new expansion content that way unless you then started grinding the best Elden Ring rune farm.
Another is that your new character wouldn't have important items like Golden Seeds, nor would they have any of your best Elden Ring weapons or unlocked Sites of Grace. This would force you to chase all these down again to upgrade your healing flask or if you want to use them for something while playing the DLC, which would be extremely inconvenient.
The only good reason to start a new character would be if your existing one is currently in the middle of a New Game+ cycle, as you'll likely find the DLC especially challenging if you play through it blind while also dealing with the increased difficulty of these.
Don't forget you can respec — here's how
If you're wondering if you should make a new Elden Ring character for Shadow of the Erdtree because you want to use one of the best builds you've never tried before, don't worry — you don't need to do that. The game actually has a dedicated respec system for this very reason, and paired with an understanding of where to find Smithing Stones and Bell Bearings, it will allow you to change your stats with ease.
My guide on how to respec stats goes over this in-depth, but the TL;DR is that you'll first need to beat the Rennala, Queen of the Full Moon boss at the Raya Lucaria Academy. Then, you'll find her sitting in her boss room, where she'll offer you a chance for "rebirth" — AKA, a respec. In order to respec, you'll need a Larval Tear; there are tons of these in the Lands Between, and you probably already have a good amount of them if you're in the endgame.
The Elden Ring: Shadow of the Erdtree DLC is finally here, and it makes one of the best Xbox games and best PC games of all time even better. It's out now on Xbox Series X|S, Xbox One, PS5, PS4, and Windows PC for $39.99.