What OP is looking for is however per definition some kind of CPI/PPI basket of goods and services, with all its challenges...
Obviously so.. but I was semi-serious.
The measure that is not volatile, does not change over time. If you really want a base measure, then it has to be something unaffected by time.
The next nearest solution is the value of unit work - manpower cf horsepower cf CPU power.
However, I wonder absolute measures have limited value? The real value of a market at any one time is exactly the demand on it. If there is no demand, there is no value; and what value there is, is only relative to what others will offer in exchange. A basket of other values might not be on target.
So, it's well known market cap means nothing beyond a contributor to utility. If you sold the entire market, what sum total would that be.. and divide by total coins.
Market value is an illusion, not just for the market cap but also for offers on the market that might get pulled. Real value is about what you can sell one single coin for.. and perhaps a simple basket beats a complex one.
Getting an absolute measure in a relative world though I suspect is more what the OP wanted.. and time is that or otherwise work. The price of a McDonalds burger is the same idea.. base work unit. Perhaps in the world of crypto a measure of complexity would work?.. The cost of a math calculation??.. link to the BTC mining cost???
[1hr x CPU] worth of BTC mining, might be stable?