This exists already. 18650 is in widespread use and now 2170 is gaining traction. Each cell is already a standard voltage, 4.2V at the top end, 2.75V on the bottom end (or lower depending on whether you want to risk permanent damage).
> Can we please get batteries with built-in protection circuits? I want to wire them in series or parallel at any voltage and have them "just work" and never explode. Bonus: please do this with solar panels as well.
These also already exist. Tesla will never use them because the BMS handles that job, which is more effective and cheaper.