the best thing to do is to ask them what WOULD work to cover it.
consider going to another shop and sondulting with the artists there as well.
its hard to give you ideas on here without a picture. i would imagine that the reason why you’ve been told most of your ideas won’t work is because your tattoo has a lot of dark ink, which is harder to cover up than lighter ink. to completely cover your current tattoo, you’re going to need a design that is larger than what you’ve currently got.