The academic literature has found a number of adjustments an investor can make to improve upon the typical standard US Stocks & US Bonds allocation. In no particular order they are:
1. Tilt the portfolio to value. (And tilt toward fundamental weightings)
2. Tilt the portfolio to smaller companies.
3. Add foreign and emerging equities.
4. Add various types of bonds, ie TIPS, foreign, high yield, etc.
5. Prefer low duration government bonds vs. long duration.
6. Add uncorrelated real asset classes such as commodities, REITs, timber, etc.