I'm going to start by talking about the various types of play experiences I enjoy, the one I don't, and then explain how that one could be fixed (most readily through WotC doing a mega-adventure or two differently in the future), and I believe improve the gaming experience of lots of players (particularly new players are having formative experiences that are unnecessarily limiting how they see campaigns).
One thing that saddens me is that what a campaign means has changed so much that we don't even use the same words anymore. I can enjoy a variety of different D&D play experiences.
One Shots: A brief scenario that lasts a session or two.
Theme Adventures: A self-contained play experience lasting for about 8-16 sessions. These start at whatever level is appropriate for the theme of the adventure, so they are as likely (or more) to be high level than to be low-level. Generally there is either no level advancement, or one level is gained part way through.
Old School Campaign: An extended series of play with continuity of characters (even though characters might leave and join and it could end with a different set) lasting probably a minimum of close to a decade, and potentially multiple decades.
There is a fourth play experience which I don't think works very well in D&D, but at the same time is the most commonly found one in modern D&D.
Theme Campaign: A play experience with a party of characters that generally lasts several months to a few years and revolves around a specific mega-adventure (whether purchased or home-brewed).
I like running all of those. However, the Theme Campaign--as designed and run--is in my opinion a poor fit for D&D. It works much better in other game systems for the reasons I'll lay out below.
1) First and foremost, my suspension of disbelief cannot handle the rapid leveling. In the course of a few weeks you go from 1st level to high level characters, often without learning or using half of your class abilities since you have new ones every time you come back from a bathroom break. It just destroys the whole believability of the settings for me.
This is a problem limited to D&D and similar games with levels and fast leveling. It is entirely absent in many systems where the characters' power level increase and new ability gain is more moderate.
2) It's based around a single prescribed storyline, so you really need to make a character based on that, but a lot of people just want to play a certain character, and don't care whether that character is a good fit for the game the group is going to play for the next year. This problem is much less likely in any of my preferred styles, either because they are short enough that it's easy to get everyone on board with the theme, or it's long enough that it should have a wide variety of experiences suitable for any characters over the course of play.
This one isn't completely limited to D&D, but it's worse in D&D because there are so many character options to try, and an assumption that you should play the same character the whole campaign (rather than having alternates for example).
3) While not necessary, people often seem to want to play such that each theme campaign is its own instance of reality. Unlike the other issues, this one is just more of a missed opportunity issue. I find worlds where PCs from one party in one campaign might actually interact with the results of what happened in other campaigns due to a more persistent world to be one of the most desirable things in role-playing. I think part of the mindset of a theme campaign (we're the epic heroes of a movie trilogy) lends itself to reality instances more than the mindset of the other play types I listed, where either their brevity (or extensiveness) screams that there is so much more in that world you haven't yet seen, and invites you to want to set further play experiences in that same reality instance.
At this point, someone is probably thinking that I'm just behind the times and don't want to update to the new styles. Inasmuch as that is true, it is due to the value I derive from the various methods not an innate resistance to change (I'm never satisfied with things as they are and am constantly seeking to improve just about everything I have to deal with, from life circumstances and accomplishments to DMing skills.)
In the spirit of improvement, here is what I think could be done to make the ubiquitous Theme Campaign work better in D&D.
1) More episodic elements with baked in downtime. The issue with the unbelievability of fast leveling from an in-world perspective can be addressed simply by making sure it takes more in-game time to get those levels. If the scenarios are set up so there are multiple parts with months or years between them, that could improve substantially on that particular problem. But generally, the adventures WotC has published for 5e haven't had that, which is one big problem I have with them and why I can't use them as designed. This wouldn't solve the out of character issue of players not really becoming familiar with their characters, but I'm not sure there's an easy fix for that that could be implemented in adventure design.
2) This one can be addressed by loosening the expectation that you make a character, play a campaign, then when it's finished repeat. Published mega-adventures that encourage you to create two characters and choose which to play at each new section of the adventure (punctuated by downtime so it's easy to justify the switch), would help a lot. Dark Sun used to have something like this, and maybe WotC could implement the idea in a Dark Sun mega-adventure. People getting into a habit of alternating what campaigns they are playing in their weekly sessions might help also, but while you could create a mega-adventure that would teach people how to do that*, I don't have the ear of WotC so the likelihood of that idea getting to anyone who has the capacity to make it happen to even consider the concept it isn't anything I have power over.
3) This is one that is entirely a choice that the group or DM makes and it would be harder to publish something that would do much. Sure, the next book of miscelania like Xanathar's Guide to Everything could have a section on running a persistent world continuity, with a lot of guidelines and such, but that's not something most people are going to get much out of (we've already determined on ENWorld that a lot of DMs haven't even read the DMG!) I think it's possible that there is a way to do this, but I'm hungry now so I'm going to finish up this post rather than try to brainstorm something.
So the moral of the story apparently ended up being that WotC wields godlike power over the future of gaming and they need to be more responsible in publishing adventures that validate multiple playstyles rather than their current approach which primarily teaches new gamers that the way to play D&D is with the fast advancement Theme Campaign model.
*The adventure would basically need to have two separate but related adventures--maybe one on the Material Plane and one in the Shadowfell or Feywild, and you'd make a party of characters to play in each of them, with the subsections set up so that you switch between them ever few sessions. This would expose people to the experience, and then groups might be more likely to split their play experience in other ways.
