Advice on the Long Game

[HoneyBadger]

I like to play SP for the simple reason that I can make it "my kind of game" to atleast a degree. The problem with MP is that, unless it's heavily roleplayed (and there's nothing wrong with that, but it's not exactly fodder for a pick-up game), the game tends to be short, sudden, and anti-climactic (and who likes that?). So what I'm asking is for advice on making a game more complex, more intricate, and-for lack of a better word-longer lasting.

Now lots of obvious things come to mind, but there are probably things I'm not thinking of, so what elements would you suggest for making a longer and deeper kind of MP game?