I assume its gonna be the SS due to the very high demand.
The SS would make logical sense seeing as it's popular and is the sole remaining perk from KF1 yet to make an appearance. But seeing as the Gunslinger has been officially confirmed, TWI may well be deviating away from releasing all the KF1 perks first. They could very well reveal in a WWAUT thread a week or two from now, that the SS is the 2nd perk in waiting.
But I suspect it will be the Martial Artist that is the 2nd perk. Reasons include:
1) There is already an MA perk weapon in Early Access: the Katana.
2) Yoshiro's "ninja outfit being not fit for work" remark in another WWAUT thread, although vague by itself, could be hinting that the perk is around the corner but isn't quite ready yet.
3) Gunslinger is all-guns while the Martial Artist (theoretically) will be all-melee, such as their namesakes indicate. With such a contrast, both perks together would actually be a very good pairing in the next content update and would continue the theme of two opposing perks being released together.
4) There is precedent for releasing the Martial Artist sooner rather than later due to how the Berserker is constantly in line of being reworked. Before the content update, the general consensus was that the Berserker was underpowered; now it's overpowered if anything. Releasing another melee-based perk gives TWI an extra reference point to help find the sweet spot with the Berserker.
5) Seeing how much extra time and effort they've put into a melee-based perk already, it's likely that the Martial Artist could raise similar concerns of being underpowered/overpowered. Although we don't know anything about the weapons or skills of the 3 unannounced perks (except the katana), recent history has shown that melee perks require the most attention in terms of balancing. Thus it would be better to get the Martial Artist out first, just in case it needs a slight rework by the time TWI release the final two perks and two maps.