OK

let's be explicit with what it is, a region. Adding Kansai in there doesn't hurt

@@ -14 +14 @@
-<gloss>Ryōtan (the two former provinces of Tanba and Tango)</gloss>
+<gloss>Ryōtan (region in Kansai corresponding to the two historical provinces of Tanba and Tango)</gloss>

I think xrefs is probably the best way to handle this.

@@ -11,0 +12,2 @@
+<xref type="see" seq="2845159">丹波</xref>
+<xref type="see" seq="2845177">丹後</xref>

These names that I have been adding in recent days are names of clusters of two or three former provinces. Sometimes such names are equal to a 
province that was split at some point (it seems most splits happened in the 8th century). I have only added geographic information of current prefectures if 
this is the case. In other cases I have only added the former province names. I still think these names are useful to have (for example, 三陸 さんりく was 
never the official name of an existing province, but it’s a fairly often used way to refer to northeastern Tohoku region. If you look up 三陸 in a Japanese 
dictionary, it won’t have current geographical information; it will just have the former province names). If you think not adding current prefecture names 
causes confusion, how about solving this issue with cross references to the actual former provinces (that do have that information)?

Could we not do this format of "two former 
provinces of..." "later bla bla" and 
instead do up-to-date geographical 
information for all of these province 
entries? I don't think it's useful to 
explain one historical province by name-
dropping 2 others.
(assuming this is a historical province, 
though that's not really clear from the 
gloss. if it's not, I don't think it's 
within the scope of proper names we 
decided we'd let into jmdict at this time)