I think there's also some middle way: Unrelated adventures/campaigns with different groups, but in the same world, at different times and places. It need not be one-night adventures only, but essentially it puts little value on long-term progression of either story or character.
I have to admit that I never tried it, but we try to motivate new players to join. Well, that's almost accomplished, the far more difficult part is to motivate them to stay...
And now the idea is to start with a weekend-adevnture where they play characters without classes which are not from the world they start in. They have nothing but their own description of themselves. No rulebooks, numeric values, or ingame terms involved here.
They talk to people and find out what's going on, and how escape, exploit, survive or fit into the world. I think the best way to describe what I mean is an example:
I have always been fascinated by a magic ittem called
immovable rod from D&D. A magic iron bar that locks itself into place at the push of a button. This is so amazing!!
Hey, look at those olympic high bar gymnasts. How much momentum they gain from swinging around some fixed bar...imagine them swinging around with something in their hands that changes to fixed to almost weightless. Warriors could climb to great heights and drop themslves in front of opponents, changing momentum from falling to horizontally by fixating the bar the moment before landing. Artists might perform magnificient performances, wearing costumes with meterlong tails. It takes a lot of artistic skill and stamina, but if you learn it from childhood, I think one should be able to even travel long distances without touching the ground. Why not, you can even sit and rest on them in midair.
And so their first adventure will start in a world where those things are rather common, and if they want to try ba themselves, they will be as succesfull as fits their character description. There will be a lot more things like that, and they can always ignore this stuff and find other things that fascinate them. No matter what, they will succeed, and never see those characters and this general part of the world again...well maybe sooner or later, quite literally.
Optimistically as I am, I hope that at least some players are interested in another session, and I think if they are, it is because they found something that fascinates them. To stay in the example above, some player might want to play a high bar gymnast who starts into the game by getting such a bar. This will be where the first character sheet is created. Esseantially D&D 3rd terms and value range, but never the classes. Like the whole system or not - I think attributes, skills and feats are handy concepts, I just don't like the rest of the books...well, maybe as some kind of inspiration, see those bars, but not for rules.
But this time, he starts being the only one having such an item...or making such use of it. Of course he first had to learn how to use those special bars - his experience is limited to fixed ones. On the other hand, to be honest - I would also let him play a kickass-champion if that's the style the players prefer. They'll also never see tohose characters againg...
But they might hear from them. Just tales from the uncle of a friend of a friend's uncle who hrad it from a blind beggar. Prophecies that foretell what the players did in another campaign, but so obscure that it will take the players to find out. The calassical motto of being unable to escape destiny

For example, in one campaign, there will be what seem to be a casual villain out to destroy the party of superheroes that defeat him after a superherostyle battle. After defeating him, they will discover that he gained some energy from some kind of "death vow" he uttered. He did it in a language they don't understand, but might be able to find someone that can loosely transcribe the words: "you not you later earlier ruin". No more answers, and silence for an adeventure or two, again with new characters. Then some hints, hidden between other independent stories with independent characters.
To make several long stories short - someday I will tempt them to to something rather reckless that by coincidence causes the total ruin of a mighty mage...they don't even notice, but in this adventure, he outmatches the party. They find out who he is and what they did and they have to escape him and all his range of tracking, scying and whatever - which they will have accomplished at the end of the adventure.
Needless to say that a slightly more talented translator whould have translated the wording of the first death vow as "you_but_not-you
particle_futur particle_past my_ruin", and the perfectly context-sensitive translator would have translated it as a curse for "someone that's both you and not you will ruin me in the past". So he had tracked the characters that actually ruined him by spell, but they escaped (or died unknown to him). For some reason, the spell "tracked the players, not the characters", and reported the superheroes decades after the original incident, during which the ruined guy still believed the characters to be alive, and believes to be facing when he plots against those superheros.
I think my players might enjoy this dawning conclusion, and i hope I'll be able not to spoil it by making things too obvious too soon.
This hook aside, the main idea of switching characters is that you don't have to bee too consistent about how things work within each adventure, while being able to weave something like a narrative thread. Characters might stumble upon the grave of a fomer character without knowing him ingame. Hear about another former character who fulfilled a dream that was once beyond the range of an adventure. Find out the the strangely patterned scorched marking on the groun, in one campaign worshipped as messages from elder stone gods, are the result of another character's magic experiment gone awry.
But whatever happens - I can always claim that it was possible only at this very time and place (and world, if need be). We could even chose different systems each time, from complex to simple, from serious to pun-ridden, from down-to-earth to greatly overpowered.
In my specific case, the friends I try to recruit stated that they "preferred games that started over each time", so I try to introduce it to them that way. If they like it, they get to learn the world by playing again in somehow similar environments. They don't want to remember stats, reidentify with a personality, or live with the consequences of what they forgot about anyway.
But I almost know fur sure that they would like the game and the system, and that is my intended way of proving it to them...
But first of all I still have to actually gather the intended players...narg, should have started in wintertime
