One thing I do in my game is use pathfinding to find suitable places for doors connecting rooms.

The process is, I randomly place some adjacent rooms, completely surrounded by walls. Then I set up a pathfinder (A*) where the cost of moving through walls is large but finite, and the cost elsewhere is tuned so that long detours can cost more than going through a wall. Then I run the pathfinder between various rooms, and place a door wherever the resulting path went through a wall.

This results in doors situated on "desire paths", so they look more or less like they were intelligently placed. It's also tuneable; you can tweak the costs to either get lots of doors and very convenient paths, or minimal doors while still having every room be reachable.

This kind of thing gets me eager to play around with game systems but the art gets in my way. I need to find a kit of generic assets that I can use to experiment with game systems development. Maybe that's why roguelikes are so attractive for this kind of thing?

A few collections of free assets you can use:

https://www.kenney.nl

https://opengameart.org

http://www.pixelprospector.com/graphics/

Sketchfab (https://sketchfab.com/store/3d-models) and itch.io (https://itch.io/game-assets) both have healthy asset marketplaces as well. Unreal also has a decent asset store and Epic has released high quality assets for free (https://www.unrealengine.com/en-US/paragon, https://www.unrealengine.com/marketplace/open-world-demo-col..., https://www.unrealengine.com/marketplace/infiltrator-demo) and Unity has done the same for some of their high-end demos. Unity asset store also has small prototyping kits for cheap. I like https://www.oryxdesignlab.com tilesets for small roguelikes and RPGs as well.

Roguelikes are one way to solve the art problem.

As someone else mentioned, the Unity asset store has plenty of assets too - some are free, and many others are reasonably priced. There are plenty of 2D assets available, as well as 3D if that's what you're looking for.

Last time I checked, the TOS/EULA says you can use Unity purchased assets outside of Unity games, too - as long as you download them through the Unity editor. So you could potentially use the art assets you buy there even if you want to write your own game engine from scratch.

You might be able to find what you need here, too:

https://opengameart.org/

Try Unity3D! The Asset store has more art than you will ever need, especially if graphical consistency is no priority

agreed

I've been to several game jams where I see amazing prototypes come up in 24-48 hours where people use free or really cheap $5-$40 asset store assets

https://www.google.com/search?q=unity+assets+for+prototyping

It's arguably one reason why there are a glut of games. Not only do engines like Unity and Unreal make it easy to get started but their asset stores have starter kits for almost any type of game. FPS, 3rd person platformer, racing, tower defense game, 2D platformer, etc there's a kit that you download and just start skinning.

https://assetstore.unity.com/search?q=category%3A90&sc=templ...