SD confirmed that if there won’t be any delays the new merc will come out around the 30th ( @Faraleth has stated it is most likely the 6th of October) so at the moment we are left with speculation on who will come out.
2 of the most likely mercs to come out are Turtle and Stoker and although what @Faraleth said (The next merc has an R in their name) Thunder is unheard of and most likely still under heavy development.
So this leaves us with the two other mercs who have an R, Stoker and Turtle.
I decided to compare the hints and see who is most likely the next merc
(Be advised, SD and Nexon are a massive troll and are not to be trusted, take everything with a grain of salt)
Stoker:
Ah my boy Stoker, long time still no see. Due the teasing to the Phoenix update a lot of people thought Stoker will come out due to heavy teasing that involved fire and this:
So it wouldn’t have came as a surprise if Stoker would’ve came out but then Shoe pushed it too hard and the teasing was clear to be troll.
So let’s analyze the image shall we:
We can most clearly see the gloves which in the third person view seem to be Stoker’s however if I am right (correct me if I ain’t) Stoker’s gloves from 1st person during CBT were same as Kira’s so it could be a fake or a design change.
What points more towards a design change is the fact that those green half orbs are supposed to be the spread of every single flame (It isn’t just one huge radius but a few so that is interesting).
Also, after the Phoenix update Stoker’s molotov effect became more polished:
https://www.youtube.com/watch?v=Dne3CknFCR4 (not mine)
So it hints towards Stoker even more now does it.
Also there are Stoker vids out there but none of them has a photo of him holding the beckhill so I just really am having a tough time judging (Thanks @Ardez )
During the new fine tuning patch the Timik (Stoker’s default primary) got a buff, could be a hint or just possible necessity.
Turtle:
Turtle was never in the game as far as I know, @Nail if I am not wrong confirmed it so he is the only merc that no one (except those QA testers) got to play. So what hints towards him? Well nothing except:
http://i.imgur.com/Dp2N6ZL.jpg
(Note: this photo is not owned by me nor anyone I know I found this photo on the internet with a description I am aware data mining is a punishable thing but I also don’t see how this is really bad since it doesn’t confirm anything I will delete it if I am asked to, please don’t ban me :# )
Oh yep this could be really pro photoshop but I kind of doubt it, if you know how to UE3 you can basically find a way to have mercs appear like this and from what I read it only worked with Turtle so this is heavily hinting towards Turtle not to mention that I am sure SD is aware of the need for a new engineer.
Also Turtles shield is in the game but seeing as all unreleased mercs have their special ability in I don’t see any real hinting. (Thanks for reminding me @Ardez )
So in the end this is what we have, while I personally want Stoker, Turtle seems likely to come out but we will have to wait and see.
Note: I will update this when more info/hints come out so stick around
EDIT: This thread is irrelevant due to Stoker’s release.
I remember when people thought that image meant they would be adding a special melee for Fletcher. 
But it would be a sneaky little hint, that they in no way planned since the beginning of the patches… but yeah, I’m a little excited about Thunder, perhaps that is why they so drastically rebalanced Fragger’s ability in anticipation of his release! Conspiracy theories! But regardless I’ll be happy whoever it is they release.