Mass can only be stored locally. Each player no longer has a "bank" of mass (represented by the mass bar--you would permanently have a completely empty mass bar). Mass extractors leave reclaim around them. Mass needs to be gathered by engineers and brought to a factory in order to spend it at that factory (or brought to mass storage building, and buildings/engineers near the storage could feed from it)
Every player would have to establish "supply lines" in order to exploit mexes. Which would create a whole new dimension in terms of setting up logistics and for raiding your opponents. It would create incentive to put factories near mass (e.g., build a factory near reclaim field to make engineers to grab the reclaim + be a convenient place to spend the mass on new units, or build a factory next to a mex so you don't have to transfer the mass anywhere, it always gets spent immediately).
There would probably need to be new commands (e.g. assist command, ferry command, patrol command, etc.) so you can set up these supply lines. Maybe it could be done just with the "assist" command, like if you assist a mex and then assist a factory, the engineer knows it has to go back and forth between the two to deliver mass.