If you're looking for a platform that offers customizable design assets and AI prompts, Items.Design could be a good choice. It offers more than 900 AI-created design assets, including 3D renders, backgrounds and abstract shapes, that you can use for free. The assets come with Midjourney prompts so you can create more assets. The site also offers different pricing tiers for licenses, so it's good for solo creators and small teams.
If you want to go more AI-only, check out getimg.ai. The site uses AI to generate and manipulate images from text prompts, including the ability to convert images to video and use an AI Canvas to extend images beyond their edges. It's good for high-resolution images and offers pricing tiers based on credits, so you can pay for what you need.