Like, a Heward's handy haversack! It's a bit of a back-end around that 10 lbs limitation.
Fails #3. If you're concerned with your encumbrance limit, why are you not already carrying the object whose whole purpose is to expand your encumbrance limit? Same goes for
bags of holding and
portable holes. Nobody leaves extradimensional storage space at home. That defeats the point of having extradimensional storage space.
Or maybe a Dragon Slayer longsword. Not always useful, but when it turns out that there's a dragon in the dungeon...
This one I can almost buy. Still skeptical that you have so many magic weapons no one has space for a
dragon slayer, though.
A Helm of Comprehending Languages when you realize no one bothered to speak Dwarven and you want to know what the Duergar in the next room are saying about you.
Fails #2. You are so worried about this possible scenario that you're willing to invest 1,000 gp
ahead of time in order to avert it? Instead of just, I don't know, putting
comprehend languages in your spellbook? It's a 1st-level ritual. You don't even have to prepare it.
A Ring of Warmth when you find out that the goblins are riding winter wolves.
Fails #3 in truly spectacular fashion. It's a friggin' ring. If you can't find room in your pack for a ring, I don't know what the heck you're keeping in there.
50 ft. of hemp rope when you're 50 ft. down the tunnel and you still don't see the end of it.
You're an 11th-level wizard and you're laying down 1,000 gp in advance to provide yourself with extra rope just in case your party doesn't have enough? Instead of preparing one of
levitate,
fly,
dimension door, or
polymorph, each of which is a phenomenally useful spell that can easily justify a spot on your list?
That +1 Club that you don't usually use because you have an axe that does more damage until you come into a room with skeletons...
I repeat: 11th-level wizard. If your party requires a magic club to deal with skeletons, you fail wizarding forever.
It is niche, but it's MUCH better than an illusion. Plenty of critters can see through deceptions and illusions and lies and suchlike. If you literally don't have the item...but you do have a sapphire...you're in good shape!
Orc warchiefs can't see through illusions. You're changing the scenario here. Which is the point; the niche is so narrow you have to pile all kinds of qualifiers on top to justify it. You want to bring a weapon to the negotiation, and the person you're negotiating with can see through illusions, and they won't let you bring weapons, but they will let you bring a mysterious sapphire radiating magic (wizard shenanigans anybody?), and you have time to prepare for this whole thing and secure the weapon in a safe place...
And Leomund's Secret Chest has one HUGE drawback: the thing could disappear forever. Woof.
Only if you're phenomenally, stupendously, mind-bogglingly careless. As long as you summon it back and re-cast the spell every 60 days, you're fine.