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.
Yeah except there's no space either for that. After Tristan and Lumis-Umbra are in the Gate, even if they put the "dots" closer together there's still not enough space for another one.
Way back yonder there was som files people discovered that had Marik(good) and Grandpa Moto as future Duelists so if what you say is true they could be the last 2.