Yes, that’s a good idea. Here are some options for reverting to the mean.
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S&P500 Earnings Yield could revert from its current 3.5% to its long-term mean of 7.2%. That would require earnings to double (with prices staying the same) or prices falling by half (with earnings staying the same).
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It’s hard to say what the 30 year Treasury yield mean is because it has been in a declining trend since 1987. But we can look at the REAL (inflation-adjusted) yield of the 10 year Treasury which was 3% (above inflation) before the markets and Fed began suppressing yields after 2000. The current REAL yield of the 10 year Treasury is 1.7% so reverting to the mean would cause the Treasury yield to rise by over 1%. That would by itself cause the SPX to drop significantly.
- The Fed funds rate could drop. The Fed is seeking a neutral rate which won’t stimulate or slow the economy and will maintain inflation at 2% while not causing a recession (or a crisis the way they did in the 2000s). They are pretty much at the sweet spot right now so there shouldn’t be much movement here. (Regardless of pressure from the President or the markets to drop rates prematurely.)
If you have a crystal ball to predict any of these accurately you could make a lot of money. 
Wendy