This one is easiest of all from implementation point of view. With image compression technologies nowadays performance should not suffer too for background image loading.
- It would be great to have Dashboard Background Image, Workspace / Board Background Images. Like Trello and almost all other tools offer that for a reason. It just helps user to personalize the tool giving it a personal touch to it. Plus it will look pretty. If you want you can have couple of tested options to choose from including just plain or gradient color background images.
- Board Colors, Label Colors etc… wherever there is need to have color in app, currently there are quite limited number of colors. Just adding more colors or possibility of giving color palate will just help user to not have duplication of colors and more importantly user will have possibility to even personalize with more flexibility with whatever color preference user has.