To be clear I don't think a story being long is automatically a problem but what really bothers me is when it wants to keep going but doesn't have enough material to justify it. Romance is where I notice this most because you can only get so much out of “will they get together?” before the misunderstandings, interrupted confessions and setbacks start feeling like the author is desperately protecting the premise.
I genuinely wish more series properly combined genres. Romance with fantasy, mystery, an isekai adventure or a battle shounen. Give the characters something substantial to do alongside falling in love, and let those parts of the story influence each other.
I think Bunny Girl Senpai is a good example. Sakuta and Mai's relationship can develop because getting them together doesn't resolve everything else happening in their lives. The supernatural mysteries provide more material while exploring the characters' insecurities and relationships. The story doesn't need to keep undoing their progress to continue.
Also Spice and Wolf come to mind. Lawrence and Holo's travels and negotiations give them opportunities to disagree, depend on each other and build trust. Their shared experiences develop the romance, which in turn gives those experiences more emotional weight.
That's what I mean by combining genres. An action anime having an occasional blush or jealous reaction isn't enough. I want the relationship to affect the adventure and the adventure to affect the relationship. Let their ambitions clash, their responsibilities complicate things, or their experiences change how they see each other. Likewise, an action series developing its relationships could be more interesting than introducing another stronger enemy whenever it needs a new arc.
Cautious Hero makes me appreciate this too. Without spoiling the ending, its serious character material changes how you understand the comedy you've been watching. The different elements strengthen each other rather than simply taking turns.
Sometimes brevity is the better choice, though. I haven't watched or read much anime/manga horror, but films like Alien and Insidious make me wonder how often revisiting a threat wears away what made it frightening. Some premises benefit from ending while they still have an impact.
But if a series wants longevity, I'd rather it expand what its characters experience than endlessly recycle one conflict. Getting together should give a romance more possibilities, rather than feel like the author has destroyed their only source of material.
Do you guys know any anime you think combine genres especially well, where the different elements actually strengthen each other?