Adventure Path, Sandbox & Megadungeon

The Big Three
Pick one! Speaking thematically, I can think of three ways to categorize the fantasy RPG campaign, adventure path, a sandbox or the megadungeon (there are probably others I am not thinking of). They are each really a campaign in themselves. It has taken me some number of years to discover that blending them all in a single campaign is very difficult (at least for me) and rarely works well. One-off session play can switch back and forth making use of each style in turn, but the full-on campaign, if it is to be more than a series of loosely connected sessions, requires more direction and more consistency.
I have not gotten to game much lately (holiday season), so my pursuit of the hobby has mainly been reading blog posts and published game materials, and streaming a few videos of others gaming. One of the RPG books I have been reading is Chivalry & Sorcery, 1st Edition (C&S). I have been known to occasionally wrap myself in the cozy idea that playing with a hyper-realistic RPG setting would be rewarding. It seems an irrational conceit, however, as during my saner moments I can't imagine finding folks who would actually play the thing. Regardless, in my present holiday season absence of gaming opportunity, I can muse on about chivalry and player character knights in a gritty realistic medieval milieu, about squires, ladies-in-waiting, minstrels and a host of serfs awaiting my noble bidding. Pouring over my C&S, Harn, and historic reference material, I immerse myself in a make-believe world where I actually referee such a game while in the back of my mind I realize that even my oldest, staunchest gaming buddies, those who once upon a time played some Harnmaster with me, today show little interest in that style of immersive play.
Taking a medieval setting as the basis of an imaginary campaign for use as an example of the "pick one" advice, I feel I would be remiss if I, as referee, tried to offer players a sandbox to roam and explore, and an adventure path full of story, and a megadungeon and expected them to simultaneously juggle them all. Overwhelmed is what I would be and confused and directionless is my prediction for the state of the group. One nice adventure path for them to follow, maybe involving a run-away princess who joins a secret organization bent on saving the kingdom from a secret plot by those close to the throne who wish to overthrow the dynasty would be plenty. By saving the princess, they help her save the kingdom. Offering sandbox-style side-quests and tempting adventure hooks not related to the adventure path could be confusing as would the sudden discovery of a megadungeon just outside town where idle adventurers could instead of saving the kingdom make their fortune taking treasure from monsters. It has not been long ago that I offered my then players just such a smorgasbord of distracting and competing campaign choices. Pick one - that is what I should have done.
It has been a long time since I have played in or ran a megadungeon campaign. The simplicity of the theme appeals to me. The dungeon itself can be very complex and should be in order to sustain campaign play. It should be dynamic and react to being explored. It should evolve with internal denizen activities that play out even without player intervention. It should remain fresh with new levels and characteristics which unlock as players advance deeper into the campaign. Reading Joseph Bloch's excellent Castle of the Mad Archmage is inspiring me to want either to create my own (always the best option) similar megadungeon or to run my version of his Castle of the Mad Archmage. I enjoy planning future games, even ones that never get played.