Theres another thread asking what will alienate you towards 5e. But I wanted to get a bit more specific...
Mike and (the rest of the Dev team) has said that he expects that D&D next will be the system that will never require a 6e, that WOTC will put out "updates" for 5e, but will never again need a complete system reboot of a new edition...
Besides being a monster huge goal and an achievement that no other really successful RPG has ever succeded in, It makes me wonder... what is the largest cause of failure of long running successful RPG systems like 2.0,3.0, and yes now PF that many are calling for a new edition for that game.
IE. System bloat.
1) Defining "System Bloat"
First, how do we define system bloat? Its a word a lot of people use when describing an older and successful rpg system that has become over burdened with rules, options, and eratta from a seemlingly endless sea of splat books. I looked for a fast and quick definition but couldent find one.
So, I'm working from the definition above. A system which has too many options, too many optional rules, too much source material to read, too many rules and erata to easily put in play without creating long lists of what is and isnt allowed in a home game, or house rules to deal with it.
2) How does system bloat happen and what does it look like.
For a perfect example of how an edition eventually gets infected with the potentially terminal system bloat disease I'd point to what Pathfinder is going through right now.
PF started out with the idea that if you make (or copy) a system that players want to play, it will be wildly successful, and they have been. First and formost they wanted to give there players a large amount of options, and since their inception, the system has grown and grown.
Every month an official pathfinder chronicle comes out that adds new feats, uses for skills, spells, magic items, prestige classes and archtypes. Not to mention books like the advanced race guide, advanced character guide, etc,. etc.. and setting books that all add scores to the above.
I couldent give you an exact number, but the number of feats is up over 250 maybe 300. each feat alters a rule in some way. Some feats are basically useless, while others are completely necessary to create a character that is effective, and still others make it easy to completely unbalence the game.
thats a lot of reading and study to be able to create a character, it also presents a near endless number of OP munchkin builds a player can choose from. Its also a tremendous amount of info for a DM to have to memorize, rule on, and implement. A lot of minor rule changes to be remembered for each character.
Its gotten to the point now where an encounter for a party above 16th level basically amounts to he who hits first, kills whatever it was, and those outcomes are mostly determined by who preped best before the combat happened.
3) How does an RPG make money month after month and still avoid system bloat?
This is the biggest question I think WotC has to ask if they want to be succesfful with their goal of "no 6e" but honestly I dont know.
3a.) What did 2.0 do right?
2.0 seemed to do very well in this in my eyes. there were no archtypes, no prestige classes really, so there wasnt a lot to contribute to bloat.
2.0 had "the complete" line of splat books which had "character kits" that while giving a lot of great background, fluff, and suggestions, most often didnt really alter much; for example the duelist. There was a solid two pages of fluff, but the only rule that changed was if wearing light armor, the duelist character got +2 ac. and that was it (far as I remember)
the 2.0 splat books also covered many genres of fantasy RPG. there were books on theives guilds, castles, books on greek myth campaigns, books on vikings, pretty much everything.
I guess 2.0 went down hill with the skills and powers line of books, though appreciated by DMs who knew how to say "no" (I for one loved the pick and choose building class and innate power rules) many suffered with the new rules and soon 3.0 was in the works, but all in all, a highly successful long running edition, and my personal favrote.
3b.) What can we learn from 2.0?
Again an assumption here, but increase the amount of fluff and background while changing almost nothing about the core system.
In my mind the core system should stand alone, if done right, there should not be any reason to release new feats, new prestige classes, new archtypes, new optional rules etc. Those are the things which people and players will be most interested in buying, but more than anything else they also contribute, and cause system bloat.
If DDN wants to avoid system bloat, and avoid having to come out with a 6e that wipes the surface and lets the new bloat begin... this is the only way that I can see the designers being successful.