Standard plastic dip or product container with lid
Hello Im a brand designer and the one most basic item I find that is missing is a standard tapered base container eg: dairy goods, ice cream etc. used in every store imaginable. Used for chip dips, sauces, dry goods you name it. The design would be essentially the same as your already pre-existing pharma creme jar but with an option to adjust the base diameter independent of the lid diameter.
See Loft object added in 4.14: https://boxshot.com/2018/02/20/boxshot-4-14.html
Like the Naturavitta yogurt samples you already have in your gallery and even a the simple tin canned food is great. The rectangle format is really useful too for real world applications. Books and cd covers are already pretty easy to do in illustrator but round and packages with rounded edges is where a illustrator user can save a great deal of mock up time!
Some images would really help :)