when I was rewatching the movies with my friend, I jokingly said "do you think she'd [alma coin] fancy the old heavensdick?" which is my prelude to say I see the heavenscoin vision as long as both parties in said relationship are in a psychosexual game and there's a shipper-understanding that: I. plutarch doesn't believe in coin's authoritarian regime but it's using her to end the current authoritarian regime, II. doing a major gamble with the odds that he may not be able to stop coin's dictatorship once it begins (because he knows it'll begin) III. knowing the best bet he has to end snow's regime is the wannabe dictator in front of him he's trying to honeytrap and his beacon of hope for his actual plan, katniss IV. coin is using plutarch's means for her cause, ready to order him to be killed as soon as she becomes new dictator and he's no longer needed and as a capitolite must die.
coin manipulating him he's not going to be discarded just like all the other capitolites (of course he will) and plutarch manipulating her by playing the role of a radical anti-snow instead of the anti-authoritarian he is. so the gist of the game is: he needs to prove to her he hates the other capitolites as much as her (he knows that no matter what he's still a capitolite in her eyes, just a useful capitolite with money and means she temporarily needs for her cause) while concealing that what he hates is snow's system and anyone who would replicate snow's system (that means coin). if he supported coin's regime ideologically, there would be no effie rescue, no letter to katniss, and even if katniss had shot her down anyway, that would be the best opportunity to become panem's president, pull the reverse lenin on stalin. it's like that meme: what do we believe in? the rebel cause! what do we want? to end snow's authoritarian regime! why do we want it? plutarch: because authoritarianism is bad! coin: to start my authoritarian regime!
she needs him temporarily for the wealth and means to rule panem, he needs her temporarily as leader of the revolutionary army to end snow's rule, even as he's sure she just wants to replace it with her own kingdom. poor man wanna be rich, rich man wanna be king and a king ain't satisfied 'til (s)he rules everything.
he takes this huge bet, literally risking everyone in the capitol, because he trusts katniss to see coin for what she is and shoot coin down when the time comes. that's the wink wink nudge nudge doesn't she look like someone we both know and despise? of his letter and she gets the message loud and clear.
a major interpretation complication I've seen around is labeling plutarch as the lenin to coin's stalin. in animal farm terms (look at the url, of course I'm an orwell nerd), coin as napoleon, plutarch as a mix of snowball and squealer. and the thing is, for coin, he totally is that. except she doesn't see him as an equal (in the farm the pigs were equals) because he's a capitolite and he doesn't believe in the new authoritarian regime she wants. they both believe in the cause yes but both for their own secondary reasons: coin – become the new snow; plutarch – no more snows. coin wants to persuade katniss into supporting her new authoritarian regime, plutarch wants to persuade katniss to release panem from a fight between authoritarian regimes. and plutarch wins because katniss believes in good.
what mockinjay does is go one step beyond the fight between men and pigs. there's no arrow to shoot the pigs in animal farm. coin's authoritarian revolution is cut short by katniss. and the abstract idea of a democratic panem is possible because both autocrats are dead. if plutarch was a lesser man, he would have named himself president then and there. an even lesser man would have tried to find another revolutionary personality to uplift as the new dictator. but he doesn't because his plan was never to see coin as dictator or be the full-time squealer to her napoleon. his plan was always the long shot hopeful dream of being part of a non-authoritarian regime, led by whoever it is, and not to be its leader. and that's what he does, he becomes secretary in a non-authoritarian regime. that's how panem progresses past authoritarianism.
keep your friends close, your enemies closer -- that's plutarch's bread (pun half-intended) and butter. for the pairing to be accurate, he would do to coin exactly what he does to snow, the difference in heavenscoin being the possibility of adding psychosexual games to the act he's putting on for her. I think that if you conceptualize heavenscoin in a non-manipulative way either way, you're not being accurate to who the characters are. and there's milk to be milked accurately there if you understand this 3D chess they're playing on each other, you know?
waiter: ma'am, I asked you if you wanted another cup of coffee.
hum, no, thank you. so the discussion there is would alma coin wanna a ride on the heavensbee train? because I don't think she would feel like she needs to manipulate him that way, she already has all the wealth and support and plan to discard him when the time comes, it doesn't make a difference. but he's a hottie and it's probably not easy to be a soon-to-be-dictator without blowing off some steam. would she feel "impure", though, for having sex with a capitolite? would it feel like spitting on the memory of her dead husband? I don't know. ultimately, it's up to her. if she initiated it, he'd be game, I think.