You can go with 3 fund simplicity or slice and dice (both camps are on this forum) either to increase diversification or to capture other risk premia. More expected return, is only expected and may not show up in your time frame and there is no free lunch re: risk/return. I am a slice and dicer and a believer of Fama-French framework so I tilt on size and value factors - more small cap weighting overall and more on value side (including international stocks), market weighting for midcap but w value tilt, and reduced large cap holdings but w value tilt. I would read the FF material and read the lengthy and frequent debates and discussion on FF on the forum w the search function, and then you decide.
I also have a small emerging markets tilt on my international portion (overall intn 25% of equity). I also have a REIT tilt to 15% of my equities, roughly 10% of my portfolio. I have more REITs as I believe they are a different asset class that only correlates on average 0.5% with SP500 index. On the fixed income side, you can do the same... stay with Total Market Bond or Intermediate Treasuries, or you can add additional classes such as HY Bond for diversification (see Rick Ferri's threads for more info/debate) or TIPS (to manage inflation risks further). I don't worry about complexity, for me managing 3 fund vs 6 funds is really moot, 6-8 is still simple in real life terms. I use TMB + TIPS (30% of FI) and have blended in more ST Bond for interest rate risk.
It comes down to your plan and whether you believe in benefits of slice and dice, or the incremental value of increasing portfolio diversification further w other asset classes. The reality is you will probably do well with either path if you follow the Bogle principles of own the market w index funds, hold a mix of equities and bonds according to your tolerance for risk, keep costs low, stay the course (no market timing, trading), and pay attention to tax efficiency. In terms of making choices based on what advisors say about the future, I'd completely ignore.... no crystal balls... you make your plan, stay the course.