A “complete” asset pack can be quite tricky. The short answer would be to have a bit of everything, like:
- UI (buttons, sliders, toggles)
- Characters (2D? 3D? Animated? include monsters?)
- Backgrounds
- Building blocks (Tiles? Sprites? 3D Models?)
- Special effects (Sparkles, Magic, Fire etc)
- Sound effects
If by “complete” you mean an asset pack that suits most people out there, then you would probably need to include 2D and 3D assets, but I think it might be better to split that into smaller chunks.
Every game is different, and depending what technology a dev is using, they can use different types of assets. The more you include, the more variety games with your assets will have. I think all the types above should help a big percentage of games, but I don’t have numbers to back that up.
These are all my opinions as a dev, I hope it helps :)