Cursed bags of tricks create tame monsters instead of peaceful or hostile. This is a “bad effect” since you cannot use them for sacrificing.

A bag of tricks occasionally spits out a candy bar instead of a monster (or perhaps along with / carried by a monster), but only on Halloween.

Dragonhide bags of tricks only spawn dragons. (Though this would include baby dragons, so it would probably at least approximate being level-appropriate).

Eating a bag of holding confers temporary intrinsic hunger, and eating a bag of tricks confers temporary intrinsic aggravate monster.

Monsters pulled out of a bag of tricks are tame whether the bag is blessed or uncursed, and hostile only if it is cursed; however, all monsters created by it are temporary and disappear after several hundred turns.

Monsters will try to loot bags of tricks along with other types of containers (receiving a bite for their trouble). On the other hand, this goes against the implicit rule that monsters know all item identities.

Applying a cursed bag of tricks may make it bite you rather than releasing a monster, making you drop the bag. This does not use a charge.

Applying a cursed bag of tricks sometimes causes it to explode, causing magical explosion damage to you while also expending all of its create monster charges at once.

A cursed bag of tricks can be applied at monsters to eat them up, which will add one charge to the bag. (The existing behavior of applying a cursed bag of tricks producing a monster like normal is suppressed.)

You can apply a cursed bag of holding (or possibly empty bag of tricks) at a ghost to capture a ghost, which can then be used to recharge a wand of undead turning.

A bag of tricks may occasionally engulf and digest you.

Assuming bhaak’s bag of tricks proposal is implemented, allowing an empty bag of tricks to function as a sack:

When the bag is recharged, don’t destroy the contents. Instead, just make the contents inaccessible until it becomes empty again. Also, add a possible random effect to using the bag while charged: it spits out one of its contents at random.

Another proposal is to place the items in the bag into the inventories of the monsters created by the bag. It could even influence which monsters get created (e.g. a monster who will receive an orcish helm from the bag is likely to be an orc).

Eating a bag of tricks (as a gelatinous cube) does something interesting, maybe only YAFM, maybe more.

You can wear an empty bag over your head to blind yourself, but this sets charisma to 6. If you wear an oilskin sack on your head, it starts a strangulation timer. If you wear a bag of tricks on your head, you get YAFM and instadeath, or possibly it just constantly bites your head.