I propose we limit Pandemonium to no more than 15 (or a similarly
small N - I'd be as happy with 10, for instance, with normal demon
levels to space out the demonlords') levels, and increase the chance
of an Abyssal rune so that doing all-rune wins is no longer a test of
patience, but of ability to play the game, and so that Pandemonium is
no longer a scum-and-you-will-find-anything playground.
Once all N levels of Pandemonium are conquered, the player cannot
re-enter Pan (all Pandemonium gates fade to empty archways,
Pandemonium cards are no-ops, or summon a random type-1, or whatever).
Pandemonium exits to the dungeon / Abyss may appear before N levels (I
suggest we increase the frequency of Abyss exits).
Pandemonium farming is powerfully reminiscent of the unhealthy aspects
of Angband / Adom / NetHack, and I think it should die. Unlimited
levels are fundamentally against Crawl's design. (This still leaves
the Abyss scummable, but not for >1 runes, and Abyss-scumming is not
as large a problem as Pan-scumming.)
Tangentially related: I also propose remembering whether the player
has ever crossed into Zot successfully, and thereafter not requiring
three carried runes, since the proposed Pan change limits the number
of runes a game can have (previously infinite).