Young investor maxed out tax advantaged accounts. How should he invest extra savings?

My older kid started his first post college job a year ago and has done a great job financially so far. He maxed out his 401K (target date fund), HSA (target date fund) and Roth IRA (VT). He lives rent free so he has some extra money to put toward long term investment and wants to open a Vanguard brokerage.

For a young investor with a long time horizon, what’s generally recommended for a taxable account? Stick with VT? Use VTI + VXUS? Something else? Any guidance on building a simple but solid taxable portfolio would be appreciated. TIA