(WARNING: Christmas Eri and Fio are no longer buyable in 1.40.0!!! 300 medals each in 1.39.0 only, no discount for you!)
About Update 1.40.0, two fortnights after 1.39.0, from the Metal Slug World FB page:
'2016.
We are opening "KING OF FIGHTERS".
This year, we will adopt a special rule of competition between teams of 10 fighters.
That's all... [R]'
I don't like the phrasing of 'special rule of competition', it's like they'll bend the rules or something or steer the game towards KOF defense. But read on...
Note the 'R' at the end. Apparently this means Rugal (a BOSS) from KOF is likely to appear.
I don't play or give a shit about KOF but their bosses are infamous for being OP as fuck.
Right now the metagame is already centered around RRLM, bosses and Benimaru, Brian, Heavy D!, Robert, King, Joe and Iori. In fact, Brian and Heavy D! have pretty much replaced Slug Armor, and Benimaru is better than Green Hazmat. If Christmas Fio gets nerfed, which is possible, it'll be the final nail in the coffin.
So this is what we Metal Slug fans waited four damn weeks for.
edit: some short notes on the new units
Eaca-B (Plane): Nostalgia blast as MS1 Mission 5 was one of my favorite missions. It doesn't really do much damage, way less than UFO Invader (also 100 AP) but its main strength is its ability to survive to fire off the rocket and consistently deal damage even in the lategame when Big Shiee is out. UFO Invader keeps dying to Big Shiee or Keesi II's normal attack in the late game. There also seems to be an early game combo with Christmas Fio that can kill Iori before he can get close. (Christmas Fio's special + Eaca-B + Christmas Fio's normal kills him if I'm not wrong, feel free to deconfirm).
Party People: The normal has strong knockback and is actually quite lagless. It beats Benimaru and Leona 1v1 (by knocking them back till they die) which is a riot, but loses to almost everything else. Like 80% of cheap units it gets owned by RRLM. It doesn't have the crazy knockback power of the Elite so don't expect to troll RRLM with it. Near-useless in lategame, you can only use the special for those laggy insignificant bullets.
Party People Cadre: Beyond horrible. Easy to kill, laggy normal attack.
Party People Elite: The normal has hilarious knockback power, and can knockback full health Slug Armor and RRLM (but not Wrath Allen or OP Brian). It can actually give RRLM openings and RRLM+RLM stackers a ton of trouble by smacking them out of their laggy attacks again and again. Loses to Red Tar Man though, but it's only 90AP so it's not too bad. The special goes HIGH and dodges a lot of crap if timed right, including all three RRLM ring lasers from the normal attack. Thus its main use is low-cost limited stalling, as it can quickly knockback most early game rush units/RRLM and use the special to stall further. I have screwed around with many Iori openers online, but they'll eventually know how to get around it. Damage is still mediocre like the normal Mars People Elite, but it's better overall. In lategame it can still get its special off for that tiny bit of damage. Is it a strong unit though? I'd say no.
Junk Tower: Just horrendous, I don't know why this unit doesn't even have a special attack. It's like an expensive sandbag, except there isn't much use for walls nowadays and there's no point in planting it far away from the frontlines in the lategame.
Maneater on the floor thing: No comment, but it doesn't seem to have that much range. Haven't seen it in Pvp. It actually does look somewhat usable based on some 1v1 videos I've seen, where people pair it with Rugal (ugh) to reflect the obvious threat of Slug Gigant.
Rugal: We shall see how op he is when more people figure out how to (ab)use him. I am wondering if he can reflect zombie vomits, because I have been using zombie vomits against Rugal users in 2v2 but have never seen one reflected. I know the reflector fails against some attacks (e.g. Crablops' special) and just gives him temporary invincibility.