I've seen a lot of people displaying opinion on 2P's as though it's fact on here and other places so I just want to get my say...
A) "Himaruya made the 2P's to be evil versions of the characters."
False, the drawings he posted of the only canon designs were titled as 'second colourings' and said to be themed on an evil organisation, but he did not suggest or state in any way that they were supposed to be evil and/or different to the original characters. At the time 2P's came out, the fandom was still riding the waves of dark! and snapped! Hetalia so the theme just carried through.
B) "They're supposed to represent the dark sides of the characters' history, you can tell by their uniforms!" / "There are canon male allies designs, the files were named 2p!"
Possibly false. The files were named 2P, but assuming that Himaruya knows that the fandom decided to call those designs that it would still be pretty strange for him to name his file that (and I'm not even sure the Japanese fandom calls them that) and not say anything directly about it. I know he's done things like that before, but it seems like he'd at least give some small clue if they were supposed to be in the same category as designs he'd posted before.
Also, just because a character is wearing a different uniform than what we've seen used in strips, doesn't mean it has a special meaning at all. Himaruya has the creative license to draw his characters in whatever he pleases, which includes alternate uniforms, without meaning anything by it.
Lastly, it is for now pure headcanon that the dark sides of their history is what the 2P's are supposed to be, based on some people's observations on what designs we currently have or supposedly have for some. Not only that but the characters have been shown going through tough dividing times in canon without a second version of themselves popping up, so even the headcanon will only make sense to the individual, it is certainly not canon so please don't act like it is.
C) "I'd like to rp/cosplay 2p! ____ but I don't know what they're like!"
Aside from a few designs, there is no 2P canon. There are some very popular versions, and sometimes people will send you hate if you try to represent a 2P the way you feel they should be. However that doesn't mean you shouldn't still do it.
There are people that only use the fanon designs for popular characters and give them different personalities than the original creator of that design did, there are even those who have their own fanon ideas from scratch. There is nothing wrong with that, there is no 2P canon aside from the few canon designs for the Axis and Nyotalia Axis and Allies. The sooner people get that into their heads the sooner people can have a bit more creative freedom with them.
D) "They're not Oc's, the fanon designs are following the canon!"
False for some. A lot of the 2P's have become Oc's of a sort, barely based on the original characters. As I mentioned, the ideas behind the 2P designs was 'an evil organisation', and the designs we're given show them darker, even in skin tone for a few. However popular designs for 2p!England and 2p!Romano show them a lot lighter in design and personality than the originals, which doesn't fit the original canon idea.
E) "3P and 4P should exist because 2P does."
False. The existence of 2P does not justify a person changing a characters design and personality, a.k.a making an OC based on an actual canon character, just because of the name. They're called 2P based on the idea of a palette swap happening in old games if two people chose the same character, but the fact is we don't really beside from theories know what they're supposed to be if anything so adding more derivatives of characters and slapping a player number on them is still making a crappy OC no matter how you try to justify it's existence.
F) "We got a Nyotalia episode, so next is 2P!"
False. Nyotalia got an episode because there was a canon strip with those characters in to adapt from. So far there is nothing like that for 2P and if there ever is, the loved fanon Allie's designs and favoured personalities probably won't be the ones that are used. Partially because Himaruya can do whatever he wants, it's his thing, and partially because he can't legally use someone elses designs without asking special permission especially since he'd, I assume, be making money off of them. And that is very unlikely to happen, so chances are even if one was made, the fandom would be disappointed.
(( Sorry, that was pretty long but I'd rather send it all at once than in parts. ))