Difficulty of the Infinite Dungeon reaches a maximum - really? As far as I'm aware, monsters continue to increase in level until they're utterly undefeatable, at which point you survive basically by avoiding them. In that sense, it does reach a maximum, but just about the only way to increase it further is to make the levels suppress teleports.

As Silfir says, this feature is already in place. I've never heard of anyone complaining that the deep ID is too easy ;)

Yeah, how is the difficulty limited? I've never heard of anything like this.

The danger level of the ID has a cap of 45. This would be reached at around level 135.

Even after 135 monster level increases, with monsters getting more HP, more attacks, more likely to punch through/destroy armour, etc. Items get wilder modifiers on them too. A balor found on ID1000 is way beyond the power of one of the D50 balors.

Deep ID is a very cool place :)

A little testing I once did says monsters seem to always have their level increased by (IDLevel/10), regardless of danger level. At 'I?' (i.e. beyond level 9999), even rats will have DV and PV above 500, and 100+ attacks per round, all of which score critical hits. And you also see a lot of mimic tension rooms, for some reason.

And at the same time, because danger level caps at 45 (48?), your items don't get better past that point.

Not sure there are DL>45 items. Except maybe Platinum Girdle.

Item modifiers do get wilder though. Gloves of smiting + 13 damage and such. Something in the deep ID affects the stats on items.

Ok, checked this once more and I confirm the wild variance in place. What else could be scaled?

Maybe the level generation algorithm could break down at deep levels, generating "wilder" setups. After all, an "impossible" room layout can be just solved by leaving and re-entering. So why not having things like rooms without doors, corridors that run in circles without any rooms, stairs generated in corridor, or at very deep levels simply random scattering of empty squares and walls?

Or, maybe, the various level elements that are missing from ID (herbs, altars, shops, etc.) could start to appear beyond level 200 or so -- deep enough to not make ID an "easy source", but available for dedicated infinity diver (darn, now I want a game mode where you can only go through ID, but have no background corruption -- essentially unlimited-time dungeon. Reminds me of Ancient Cave from Lufia II...)

Other weird things at low levels:
Unique monsters -- like artifact guardians but without artifacts
Absolutely anything could appear, including elemental grues or jackal daemons

Have you been to the deep ID? It's already quite weird and fun enough.

Always liked that idea too, although it sounds like something for ADOM Deluxe.