Some market strategists have discussed changes to the 60-40 mix to account for a negative outlook for bonds, since interest rates are low and probably close to their bottom in the current down cycle for the economy. If rates edge back up even a modest amount, bond prices will decline.
To counter declining interest rates and low yields, investors could consider the Horizons Balanced TRI ETF Portfolio (HBAL-https://horizonsetfs.com/hbal) which offers a 70-30 stock bond mix with potential tax benefits.