Tristan is most likely our last, or second last DM character, depending on whether they will give us one more character or not.
There's just not enough space anymore for the "dots" in the Gate for DM characters, at most it can only contain 2 more "dots". Tristan will occupy one of them since he's not in the Gate yet, so there's only 1 space left. Whether that 1 space will be used or not, we still don't know.
There was a super-ancient "theory" that surmised that there wasn't much space left for more "dots" on the gate that correspond to duelists. Of course, this was quickly proven to be a stupid assumption since they can, you know, just move the dots closer together when they add a new character, which is exactly what they did, and still do.