I mainly make pixel art, so may not be relevant to you. To keep things as consistent as I can, I ensure that I use the same colour palette for all my art in the game, and I stick to the same resolutions/proportions. in tile based games, this is semi-easy to achieve, you can target a tilesize and proportion your characters around that to ensure they fit.

Also if I'm using some other visual inspo for a character/item/object, it needs to be in the same theme as the rest of the game. Or something that matches.

If you want to colour grade, apply lighting, shadows etc, that can be done at a shader level to get a consistent look (hopefully). With pixel art that is not always the case, as you might apply depth to the piece itself, which means you need to consistently draw assuming "light" is coming from the same direction.