How to hand over value of field 1 to class array of field 2 programmatically in .theme
Context between template variable arrays is an old problem already
  known in D6 and 7 on theming level. What about D8 now? Any light on
  the horizon?


No, there is no light on the horizon, quite the contrary. Because of the concept of nested render elements, that are rendered independently. In D8 it is getting far more complicated because of the dynamic cache. And it wouldn't work at all with the plans for the future with placeholders and lazy building.