If you manage to avoid dying by sinking in lava by freezing it solid, you are stuck to the spot for a few turns and when you escape, you leave your boots behind (probably either destroyed, since they cannot be recovered from the solid stone, or buried, and recoverable by digging a pit on the spot).
A shopkeeper who hates you (e.g. for having stolen from them) will be reluctant to serve you and will insist you wear a shirt and shoes before you enter.
In variants with racial shopkeepers, the shopkeeper can also hate you on principle without you doing anything wrong. In variants with Convicts, their striped shirt could be unacceptable for purposes of having a shirt.
Two new objects, spiked gauntlets and spiked boots. Both are nonmagical and made of iron. Spiked gauntlets give a large damage bonus to unarmed combat and also give more AC than standard gloves. Spiked boots give a large damage bonus to kick attacks, and allow you to walk on ice without slipping, the same as snow boots do.
New object type “boots of burden”, magical boots which increase your carrying capacity by 50% when worn.
A realistic implementation would mean the boots cease to work when you begin levitating or flying even if you are still wearing them, but this may be too tricky to implement given that the carry cap change may need to prevent the hero from leaving the ground in some circumstances, and the game needs to concisely tell them why.
Boots of fleeing: they give bonus movement points based on how many turns ago your last offensive action was (offensive actions are things that would fade an Elbereth underneath you). For the first several turns after combat they grant no speed bonus, then gradually scale up to full speed. This means that remaining in combat grants you no speed advantage but lets you travel very fast if you avoid it.
Cursed boots that you have worn for at least 300 turns should emit a stinking cloud when finally removed.
If you are wearing iron shoes, you cannot be forcibly knocked back to other spaces, unless you are levitating.
In variants that have mud or muddy swamp terrain, wearing mud boots confers a benefit on that terrain (depends on what the terrain does).
Boots of earthquake: when you stomp the ground with them (by kicking downward), an earthquake happens. However, this spends 1 point of enchantment on the boots (not going below 0; if they are 0 or less then no earthquake happens.)
Stomping boots. While wearing these, you may instakill and move into the square of all small monsters in the a, s, and x classes.
Thick woolen socks, worn in the boot slot. Give no AC, but provide cold resistance.
A type of boots, perhaps high iron boots, that protects fully against xans.
If the metal spellcasting penalty were to be removed entirely for one armor slot, it should be the boots slot, because your feet are very distant from the magical casting part of your body.
Gauntlets and boots of reflection.
Boots of shock resistance, which have a thick rubber sole to prevent you from being grounded.
Note: several other people have commented that since such boots would prevent you from being grounded, you would actually take extra shock damage from other things, like touching grounded metal objects.
