To go over again the information pointing towards mesmer, in case anyone isn't aware already:
1. After the ele, Necro, Ranger, and Warrior reveals, there was "at least one class returning, one based on a previous one but different, and one completely new class".
2. The thief was not called an "assassin" because that was considered too associated with Cantha, and the monk was removed/folded into the guardian to remove a class associated with dedicated healing, leaving a mesmer as the only non-announced Original Guild wars light armor profession.
3. This piece of concept art. When the elementalist was released, and warrior mentioned in a description, similar pieces of concept art led to correct predictions of all the other classes. (including the engineer, which had almost nothing else to point to it)
4. A somewhat recent announcement has said something along the lines of "mesmer lovers will have a home in guild wars 2".
5. Lots of equipment seems to resemble what Original Guild Wars mesmers had.
However, since speculation is always fun, Why not take some guesses on the off chance that Anet changed their minds/were confused/etc., in the above information, and the final class is not the mesmer?
I did have some more ideas in the past, but am blanking on them while writing this post, so for now:
Speculation # 1: Ritualist
Unique mechanic: Spawning power. In addition to carrying utility skills, ritualists could place certain permanent skills (spirits, ashes, infusions) into special skill slots, the amount of which would be determined by the "spawning points" the ritualist had. Once in this secondary bar, the skills would be a permanently active (infusions "on" all the time, ashes in use permanently, spirits constantly alive instead of on a time limit, etc.). The amount of slots taken up by skills would vary with the power of the skills when permanent (so, for example, an elite spirit might taken up all the spawning skills, as it would be extremely powerful when permanently on, while a quick cooldown infusion might only take up one slot.)
Spirits: Summoned creatures that do their own thing. Unlike GW1 spirits, these can move. They have a lot more functionality than creatures from other classes, being able to heal, apply a variety of conditions, etc., rather than just attacking.
Spirit Destruction: Like necromancer and guardian skills, ritualists would be able to destroy their spirits for additional effects. Skills like rupture soul, consume soul, etc. would come into play here. Other types of secondary summon based skills would also be available. Spirit destruction would either not be available when in a spawning slot, or would be usable on cooldown with the spirit itself respawning instantly, or staying alive through the use.
Ashes: Would replace weapon skills with powerful and/or unique abilities. These abilities would have limited amounts of uses if the ash skill were in a utility slot, but would be usable indefinitely if the ashes were in a spawning slot. Only one ash would be allowed in the spawning slots at a time, for obvious reasons (Though they would not take up all slots)
Spirit Infusions: Could be turned on and off. When "off" would provide minor buffs, chances for effects, etc. When on, would have more powerful buffs. Infusions in utility slots would have a cooldown longer than the "on" duration, infusions in the spawning slots would always be "on".
One-hand: scepter, dagger, mace
Off-hand: focus, dagger