I saw some topics on FAF forums about making mass fabricators more useable. Most ideas were about reducing cost of mass fabs or energy cost of its usage(or increasing mass income), but i think, 1 mass for 150 energy is balanced and fabs have fair cost. So changing anything may force a lot of tutrling.
Main idea:I suggest to remove mass income at all, reduce energy outcome, for example, to 75 and make a huge buff to its adjacency bonus (i think 60% for surrounding buildings). This would mean that mass will come directly into units or structure upgrades. (My numbers here may not be correct)
Developing this idea: I suggest to add to mass fabs ability of mass storages (increasing mass income of mass extractors) or add to mass storages an upgrade to t2 mass fabs (making mass fabricator a t2 stage of mass storage, but that would mean to change the design of fabs or storages)