cheapass batman in Ravnica

[madd0ct0r]

I'm playing a thief in Dnd 5e, set in Ravnica, which is basically a sprawling high fantasy city world dominated by guild politics.
To more precise, I'm playing a golgari swarm sewer elf, part of the guild responsible for waste cleanup, composting and all sorts of process's that involve the words 'dredge', 'rot farms' and 'feed the poor'.

rulewise, the thief is batman - I get a bonus action each turn to use something non-magical from my belt. I've also got a poor memory, so looking to limit myself to a handful (five) swiss amry knife type items. I know one is going fo be some flasks of chilli oil. Useful for gereasing hinges, setting slipperly stair traps and just for throwing in people's faces.

What else non-magical things can people suggest?

[Formless]

How about a snorkel? Remember, the undercity of Ravnica is mostly sewer, but also parts of the city is floating on the oceans and most people don't even know it. Definitely expect some places to be flooded, possibly with seawater. Also expect to make saves against disease, because as we all know, swimming in sewage is bad.

[Solauren]

Get a hold of a PDF copy of 'Aurora's Whole Realms Catalog'. There are lots of useful tools in there.
Conversion should be easy.

[Feil]

Sadly, most of the Batman toys in 5e suffer from low fixed DCs, high prices, and/or weak effects, unless your DM cares to houserule them more generously for you.

Rogue specific things: If you pack a bunch of 30-ft ropes with grappling hooks at one end, you can combine them with your fast climbing to swing around like Spiderman until you run out of ropes. Or get 5-ft lengths of chain with handcuffs at both ends. Combine with Sleight of Hand (also a Bonus Action) for more Spiderman tricks.

General Adventuring Gizmos:
A bag of fine dust (chalk, flour, etc) is a low-cost countermeasure to invisibility in a confined space, and may also help with finding secret doors, identifying traps, following air and water currents, and so on.
You already mentioned oil, useful for a variety of purposes.
Environment-appropriate camouflage blanket.

As a general memory tool, one great way to make your tool set more accessible without loading it all into mental RAM whenever you play is to make little note cards for each of your tools and weapons, and deal yourself a hand of these "playing cards" to play from during the game. Also a nice thing to do as a DM. It feels more real to get a notecard with an item written on it than to just be told you get the item. Especially handy if you're passing items between players and don't want to accidentally lose or duplicate an item based on players thinking both or neither of them have it.

[Darth Lucifer]

Comic book Batman has a lockpicking kit and glue globules in his utility belt in the comics. I don't know about the fantasy equivalent of the latter (maybe some kind of tree resin or secretion from a creature) but tools for bypassing locks are pretty standard issue in D&D. The glue is used mostly against one or more opponents and it makes it harder for them to move and fight.

Items like marbles or caltrops might be useful, I think Batman has also used those in the past. And if you can get something like a cable saw, not only can you cut through things, it can double as a garrote.

[Formless]

Comic book Batman has a lockpicking kit and glue globules in his utility belt in the comics. I don't know about the fantasy equivalent of the latter (maybe some kind of tree resin or secretion from a creature)

In earlier editions of D&D the Tanglefoot bag was a standard alchemical item anyone could buy, although it was relatively pricey at 50gp per bag. For whatever reason it didn't make its way into 5E, perhaps because some people felt it was a bit OP at low levels. Or maybe with the dumbing down of the rules they couldn't figure out how to make it work in 5E.