This all sounds like a market opening for a top notch AM battery
There is an opening, but it hasn't happened. Admittedly, I haven't thought much about why.
I haven't torn into any tool battery packs, or gotten familiar with their configurations, but do know a high quality, high capacity 18650 like most of them use goes for between $5-10 retail. A basic 2.0Ah ~18V pack needs five cells.
But the batteries aren't necessarily complex, and to my knowledge, there haven't been any DRM shenanigans attempted by the tool makers, like the printer makers have done to discourage aftermarket ink/toners.
People may just be wlling to only buy the brand names, have no hassles, and when they go on sale, like around now, they're can be a decent, if not pretty good, value.
For a third party to try to break that market, especially one dominated by a few large players controlling most of the tool brands, might be too difficult to consider.