I tend to find roleplays involving predetermined plots run by a GM and played-by-post on a forum usually ends up like this. The fact of the matter is, when you've got a few people writing multi-paragraph posts on a plot, the amount of detail going into covering that plot will happen very quickly. Give this a day or two of consistent posting and...taking into account the hours (sometimes) it can take for someone to formulate their detailed post, and the ins n outs of a plot can be explored by the players much more quickly and thoroughly than the GM usually anticipates.
For this I find that the most successful RPs I've been a part of have all been large-scale sandboxes. Any RP featuring a persistent world in which multiple people RP have been the only RPs (is this my fault? Maybe) I've done that always had something, somewhere, to offer the next day, month, or even year after. An emergence-based (people make the stuff, and it all "comes together" in a hodge-podge) sandbox will probably always have the most longevity in play-by-post roleplay. The Multiverse on this site is an example.
Long stories are possible and usually are the aim of most roleplays, I think. The issue is, when you're playing it on a system in which creative writing is the sole means of playing (without a system of dice, figurines, dungeon maps, what have you), it places enormous, often unrealistic demands on the DM in terms of motivation for effective management and the creation of story lines and plot devices.
I think, in short, trying to DM a long-term storyline that's your own little RP is usually biting off more than you can chew.
Creator of the Fantasy Sandbox roleplay.
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