As a DM and a player, I tend to focus a lot on voices. To me, how a character speaks is the key to cracking their personality.
Also note - a change in voice can also be key in allowing yoru players to know without having to ask whether it is the NPC speaking, or the GM speaking. So you can have:
Evil Wizard: "And now, I shall disintegrate you all!"
GM: Roll for initiative.
and not
Evil Wizard: "And now, I shall disintegrate you all! Roll for initiative!"
The difference is between a combat starting smoothly, and the gnome PC trying to strike up a dice game with the villain in the middle of a tense scene.
Also note that there's much you can do with voice other than an accent. You can change pitch. You can change speed. You can change word choice. You can be harsh or smooth, and so on.
I’ve never done any sort of cosplay at a session, but have occasionally thought of having a dress-up session, with extra rewards for people that do so.
Costuming for a tabletop game usually doesn't add a lot of value. You are primarily sitting, so much of your costume is out of sight anyway. And the GM can't do *that many* costume changes and not be called "Barberella" forever after. Costumes are also usually more awkward and uncomfortable than street clothes, which gets to be a distraction. They are more an addition to live-action play, imho.
A variation on the music theme is the party mixtape. We had a tradition that ran this way: Each player suggests one song that reminds them of or otherwise signifies each of the other characters in the party. Then, from the list of nominations, the player picks one or two that they think matches their character best. These become the mixtape. You may include the GM and the BBEG in this, if you want. You can then have this running in the background during sessions.
And let me tell you, when your character's battle tune comes up when you're in a major combat scene, it is awesome