Amy Coney Barrett is the best option. However, the time to nominate her is now(assuming RBG steps down before the 2020 elections). The Republicans will lose senate seats in 2020 and will probably lose majority in 2022.
Not happening. Unless "steps down" is actually code for "dies."
She won't voluntarily step down unless there is a Democratic president. She is 87 years with many health problems. Is it possible that she can be removed if she is not able to do her job?
The precedent for how to deal with a Justice who can't do the job any more was set in 1924, when Justice Joseph McKenna was showing obvious signs of dementia and his colleagues tried to persuade him to retire. It took a couple of months, but he finally gave in and handed in his resignation on Jan. 5, 1925. In the meantime, while the other Justices were aware there was a serious problem with his mental health, they concluded that whenever they are split equally, four to four, and McKenna would be the deciding vote on a case, they would postpone the decision on that case until after McKenna retired. So the point is that the other Justices know how to deal with the situation when a colleague is suffering from a mental health problem.