Inspired by this thread:
https://www.alternatehistory.com/discussion/showthread.php?t=144730
There are exposed some of the reasons why Rome did not manage to conquer all of Europe, failing to become the European equivalent of China: not enough cultural depth, geographical differences, heterogeneous cultural background and issues in integrating different nations etc.
One of the most interesting arguments was that Rome was not native of the North European Plain, but was Mediterranean, like most middle eastern civilizations.
This caused Rome to focus more on Persia and less of Europe. This is a different explanation than the economic one (e.g.: Northern Europe is poor).
For instance, if a Gallic Empire somehow managed to expand and consolidate in Northern Europe, it would be more likely to focus on the East (following the plains), rather than South.
This is an idea that i managed to patch up, but has someone any more info or articles to sustain this thesis? Also, i need sources/books on pre Roman Celts and Dacians, i heard they were quite developed but never got my hands on the data.