What FLAILSNAILS does is advertise a GM's willingness to include characters from more than one system, but it does not promise that a GM will use the *mechanics* of any system other than whichever one that GM normally uses. The limits of compatibility and degree of conversion are up to each GM. What the Monte Cook article sounds like (although it's ambiguous) is a promise that each *player* decides which mechanics will be used with a character, and the GM must comply. The article is claiming that 5e will include methods of making multiple subsystems work together, as opposed to methods of translating multiple subsystems into a consistence system on the GM's side. (Also, I note that Monte specifically states here that this won't be a new version of D&D, but it will be the standard in use right now, so presumably there are no new mechanics, only methods of translating 0e, 1e, and 3e style characters into 4e. But that's beyond the scope of what I want to address.)
I don't think you can or should support multiple mechanics based on player demand. Translating different levels of character complexity might be another matter. The way I see it, you can define a sliding scale of character ability, something like:
- Ability Scores,
And different scale for character classes:
- Basic (two class abilities,)
- Advanced (multiple but minor ability packages,)
- Heroic (amped-up abilities,)
- Epic (abilities plus power.)
(The numbers sort of match editions, but that wasn't intentional...)
What the GM does is tell players one or two breakpoints for each of these scales. Anything up to the first breakpoint is allowed without modification ("I will be using the six basic abilities plus skills".) Anything beyond the first breakpoint up to the second will be allowed, but will be modified and may require a handicap ("I will allow feats, if I understand what they are supposed to do in terms of the setting, but they will be limited to adding a bonus based on one of your ability scores.") Anything beyond the second breakpoint will not be directly allowed ("If you have psionics or any at-will or encounter powers that can't be translated into something less powerful, they won't work directly in my game world.")
The word I want to emphasize is "directly". For anything beyond the first breakpoint, the key is that the flavor text of the ability or class is what gets used, not the mechanic. Stuff in the middle range (1st to 2nd breakpoint) will be turned into mechanics by the GM, but not necessarily the mechanics the player expects. For example, I allow backgrounds or secondary skills (per the DMG) in my campaign, but not proficiencies or 3e-style skills, so if someone wanted to port their 3e character into my game, I'd ask what the various skills mean, then assume that's just part of their background and ditch the d20 skill roll and high skill bonuses. And stuff beyond the second breakpoint? You can translate it into flavor, but I'm not applying any special mechanics to it. You have Super Mind Control? If you win a fight (standard attack roll) or persuade an NPC (standard reaction roll,) feel free to say it's because you were able to control their minds.
One of the things I'm thinking of is: if I adopt FLAILSNAILS for my real-world campaign, how would I handicap feats and classes with too many abilities for my taste? Some of the Advanced-level classes I could see condensing the power list, probably the way I did the druid (bundling lesser powers into a replacement for Turn Undead, using the same mechanic.) But for others, and for anything beyond that level, I'd probably limit the total class ability in terms of hit dice and require one handicap per "hit die" of power. For example, you come to me with a special magical fighter, with full fighter ability plus at-will Detect Magic, Read Magic, and Detect Invisible, all at 1st level. I'd total the spell levels and treat that as four extra "dice" of power. That character would need at least one handicap worth four dice (maybe a periodic uncontrolled polymorph to a weak form?) or four handicaps worth 1 die each. I haven't worked out all the details on this, other than equating magical/psychic powers to spell level and equating damage dice to hit dice. Maybe I'll divide the extra "hit dice" of power by the character's actual level, so the same magical fighter at 4th level would only require one handicap.