I'm quite interested to see what role they give RDM. Traditionally, RDM had no defined role, he was just a Jack of all trades with some sword skills, some black magic and some white magic, but within the clearly defined role system of FFXIV, that doesn't really fit too well, especially the white magic part. He could end up being a melee version of bard/machinist, with like support abilities(with maybe minor healing), ranged spells, debuffs and sword skills, and I think that's what probably would make the most sense. Maybe they'll add a 4th more defined role with supports and rework bard yet again. I don't see him as being a healer type and tank type is obviously out(or is it... well most likely out).
There is also the question of how many classes they're adding. In Heavensward, while they technically only added 3, Ninja was added ahead of time a few months prior, so in the end we got 1 melee DPS, 1ranged DPS, 1 tank and 1 healer. Will RDM be the DPS class that fills both melee and ranged slot, or will they add another DPS class. Samurai as some sort of counter tank makes some sense, but could also be melee DPS, leaving one tank slot to another class. As for the healer, Dancer makes sense too and fits pretty well theme wise. Although who knows at this point. Probably won't know for sure until like February or some shit, they like teasing shit for a while.
Edit: Assuming they follow the same pattern as last expansion, next Fanfest they'll reveal Red Mage and tease the next class(last expansion for the initial heavensward tease, YoshiP had a T-shit of Batman for Dark Knight tease, then the next fanfest they revealed Dark Knight and he had a James Bond t-shirt as a Machinist tease). Astrologian wasn't teased ahead of time I think and was just revealed along with Machinist.
Oh and there's also some job teased during 3.4 story, haven't played since 3.0 so don't know about that but guess that'd be another class. I probably should resub and go through the story before the actual trailer, since last time it spoiled a bunch of stuff if you hadn't been keeping up.