It's looking like Adventure Time has fallen back to being just a kids' show like I feared. My guess is they went too far out to make money and were lectured by the network that they exist to sell merchandise and (eventually) movie tickets.
Season 6 had some of the subtextiest tv episodes of all time - Walnuts and Rain, Graybles 1000+, Chips and Ice Cream, Friends Forever - as well as some less guardedly profound ones, like Astral Plane, Jake the Brick, Breezy and The Tower, and the thoroughly insane Is That You? and Food Chain. It was basically wall to wall fantastic till that somewhat silly last week - continuity has rarely been their strong suit, with the season 6 2-part opener maybe excepted.
The Everything Jake episode was more intertexty-metatexty, tearfully so for Futurama lovers unthrilled by the Simpsons sendoff, and in a way which may have inspired the even more intertexty-metatexty Rick and Morty episode where Rick dates a parody of the Borg named Unity (because Com-). Chips and Ice Cream was both subtexty and metatexty and may have been an inspiration too, but of course Harmon has been at meta- for years. Not reliably entertainingly, but mostly.
I'm never sure if subtexty is valuable in itself or just comes to feel that way to those catching it. Probably that depends on the justification - for how well hiding allows important but elusive, unpalatable or hard to digest truths to slant into the unsuspecting.
The Leftovers did this nonstop. The Walking Dead has since season 4, though more heavily (often maximum heavily) in "town" episodes and bottle ones. I think Better Call Saul caught me completely by surprise with its - you find out the whole thing amounts to a damn near pure allegory. I'm not sure if you could quite say Breaking Bad was aggressively subtextual - I mean, it totally was, but in ways complementing its overt message. It was sneaky about how well and exactly it was saying what it was saying, I guess you could argue, but it was more expanding on a plainly conveyed mission statement than Trojan Horsing anything in. Could this be part of its greatness? Erasure of the line between allegorical content and vehicle? An efflorescent relation of micro to macro? I love it too much to not sound ridiculous when talking about it.
Game of Thrones carried off just the one significant subtext strain - the bug stuff and that to which it pertained. I mean, it tries, but mostly either trips over itself or falls into the eye-rollingly obvious in paradoxically moralistic ways.
Not a big fan of how Fargo employs it - the movie basically didn't, after all, beyond the plain message that criminals are selfish and short-sighted, those keeping justice alive patient, kind and methodical. Season 1 was riffing off No Country's message but added nothing crucial. 2's trying to left-wingify it, but doesn't seem to know how beyond an avalanche of standoffs and massacres. Hopefully it's up to something long-term sneaky like Saul and I'll eat those words.
The shows that do it best tend to either do it foundationally or exist in universes where pretty much anything can happen (restricting the latter to one-episode triumphs, mostly). Recent Walking Dead is maybe the one exception, but finds ways to seem retro-foundational (maybe drawing on something foundational in its source material mostly tv-ed out of seasons 1-3?).
For the foundational ones the tough trick is to stay character-based in an engaging way. I think The Leftovers ran into trouble with that, at least as a whole - too much fun bled out, and I don't think sadness was the issue (sad can entertain best of all, in the right hands) but the stretches and repetitions needed to bridge the stories of overly-similar or allegorically unrelated persons. It takes remarkable deftness to keep these gaps and arbitrary crossings thereof feeling natural, though it's clearly doable. I think a lot of the solutions found in that show are pretty ingenious, even. But enough small acts of papering-over require a full-room sheet to paper over THOSE - you need the epicycles to have to exist, not just laboriously and technically get to. Better to restrict things to just a few characters, like Saul, or find ways (and space) to allow the characters to be more than what they represent like in Breaking Bad. Jesse barely represented anything, after all, past helpfully keeping onscreen from the first a reminder that badness can be thrust upon us, to compare how we take each new version of Walt with. But within that wide set of bounds Jesse could be all kinds of other things - like an audience surrogate, comic relief, moral compass etc. And the arc of better-and-better he's allowed to follow isn't necessary like Walt's contrary one, so gets to happen in a more relaxed way. Keeping Walt both human and arc-bound I don't even know how they did. Well, I do: they hide his real bad decisions behind fake ones, ones where we agree they look bad deontologically but which we can't quite condemn - the real ones in fact never happen, but their results do. So at every point he gets to be an Everyman who happened to wake up in an impossible position. That's the main arc, anyway, but in the meantime he commits a number of sins that look small but that in fact are much worse, little stretchings-out into each new role he didn't ask for but is clearly stuck in so why not enjoy it. Yeah, masterstroke one was blur out the first decision point; two was the long garden paths where each last bad major faux-decision locks him into multi-episode, at one point multi-season damage control; three was to have the real breakings (cracks?) occur on a politeness, sometimes nearly an aesthetic, level. Four was to have him seem worse than he in fact was, once actually and (seemingly, amazingly) suddenly "bad" - so that we're with everyone else in hatred of him (him included!) hence have to be won back. Which he does, and not by actually becoming less bad than he's been! Seriously worthy of study, how they did all that. The Saul move was worthy of those. It's hard to imagine how Gilligan and co. will manage to pull off a second with Saul, but at this point I don't think we can doubt *that* they will.