All spins are based on the forward scratch spin which is really not that difficult to master (after one does a few thousand of course!). Sasha's was great because it was well centred - a must for a great spin - and she added variety to it by inventing a new arm variation.
Unless one comes up with a variation - such as Sasha did - spins are pretty much the same. It's like trying to reinvent the wheel. Pair skaters sometimes come up with a new variation.
I guess in answer to your question "how difficult it is to do new spins" would be the same answer as to how difficult is it to do new jumps. It's a challenge!