The AGA ruleset states the following:

My question is: What if white passes first. Then black passes. And then white plays a stone on the board. Would that be an illegal move by white?

No, just unusual.

I thought if both players pass consecutively the game is over, and if one player wanted to place a stone after this they’d have to allow the other player to play first.

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AGA rules explicitly don't work like this, replacing it with the players passing until white has passed last and there have been at least two passes. After that scoring works as 'normal'.

The point is that it means both players must have made the same number of moves (either on the board, or by passing), which along with pass stones allows the ruleset to give the same result with territory or area style scoring.

AGA rules use area scoring. They give the same result with territory or area counting for area scoring. The pass stones and White passes last enable that the territory counting determines the area score. The last move is a white pass that belongs to a succession of passes, so that Black and White have made an equal number of moves (regardless of who made the last play on the board).

And that third pass must be DIRECTLY after the two consecutive passes for the game to end, right?

Not directly. There is also the phase of optional agreement for removals.

The reason why I am asking these questions is because I am working on a ruleset of my own. In this ruleset you have to play dead stones out. And I think I realize now that you have to do like this:

black passes - white passes - game over

white passes - black passes - white passes - game over

white passes - black passes - white plays - game continues

white passes - black passes - white plays - black plays - white passes - game continues

white passes - black passes - white plays - black passes - white plays - game continues

Trying my best to put this concisely... Howsabout:
The game ends when white makes the second of two consecutive passes.

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Too complicated. With AGA style rules, if you want to play dead stones out, just say that at the end of play all stones on the board are considered to be alive.

If you want to get different results from AGA rules for certain Moonshine Life type situations, may I suggest using the Ing style of ending play? The cases where you need four passes (maybe five if you combine AGA and Ing) are rare, but they were important to Ing.

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That's Japanese rules. Under AGA rules play simply resumes.

_________________
The Adkins Principle:
At some point, doesn't thinking have to go on?
— Winona Adkins

Visualize whirled peas.

Everything with love. Stay safe.

What do you want to achieve beyond what is already achieved in rulesets such as

http://home.snafu.de/jasiek/simple.html

? Two successive passes, no life and death in the rules, simple!

If, in a new ruleset, you have to play dead stones out, would it be best to FORCE white to pass an additional pass instead of giving hir an option to play on the board? I mean, what is the purpose of letting white play on the board again? Black isn't given that option.

For area scoring using territory counting (yeah confusing), it evens up the number of moves each side makes making the two counting systems equal in a vast majority of cases.

later... I think I slightly misunderstood the question. Are you suggesting black pass force white to pass? That could be bad if a point was missed by black. I've had that happen... so white should be able to take it.

But white didn't take it the first time. So white has already wasted hir chance.

Ok, so you're suggesting a rule that when black passes after white, white is forced to pass. I guess the only issue to me is that it's really not needed for any purpose. White will pass and end the game. (until Bill Spight comes and finds some odd endgame scenario )

I thought you were suggesting white plays, black passes, and white is forced to pass which wouldn't make any sense.

Hehe, when I think about it, it doesn't make any sense to force white to pass but it doesn't make any sense to let white play on the board either.

Or?

The first oddity that comes to mind is white passing after black wins a ko fight. Black does not need to fill the ko because his pass will end the game, and all stones on the board at the end of the game are alive. In theory this could lead to a situation where black has more ko threats than white has legal moves, so a one eye+ ko shape group lives.

The Ing 4 pass procedure prevents this. It is a bit complicated, but if you are going to end play by consecutive passes instead of by agreement, then, IMO, it is the best procedure currently in use, except for the difficulty in distinguishing between fighting ko and disturbing ko.

Traditionally, games ended by agreement, but the reason why was forgotten and that caused a problem when Takahashi did not agree to end a game vs. Segoe. See http://senseis.xmp.net/?TenThousandYearKo%2FRulesCrisis

To avoid the kind of Moonshine Life position that Mef suggests at the end of the game, the game cannot simply end with two consecutive passes. Ending play by agreement worked for centuries, and I think it can still work. What you need is to provide for the game to end without agreement if only one player wants play to end. (Under tournament rules, you also want to provide for play to end even if neither player wants to.)

One possibility is to say that if Player A passes in Position P and suggests ending the game, then if Player B passes the game ends. If Player B does not pass, then if play returns to Position P, the game ends. (Under these rules a pass lifts a ko or superko ban. So in a Moonshine Life kind of position say that Player A takes the ko, Player B passes, Player A passes and suggests ending the game, and then Player B takes the ko.)

See also http://senseis.xmp.net/?MoonshineLife , http://senseis.xmp.net/?SpightRulesII , and http://senseis.xmp.net/?SpightJapaneseStyleRules (Article 9).

_________________
The Adkins Principle:
At some point, doesn't thinking have to go on?
— Winona Adkins

Visualize whirled peas.

Everything with love. Stay safe.