Czechoslovakia and Poland had a disputed border region, so a "what if it came to fisticuffs" in 30-38 might be worth a try. (Poland did take over some parts, just before themselves being invaded). Just find a date where both sides have "useful" AFV and you should be good to go for a short length campaign.
Hungary also had a border region in dispute in the 30s - but who with I cant recall (Just something from playing Hearts of Iron there). Hungary has it's own tank models as interest factor. It may have been a slice of Romania they desired?.
Also, Romania and The USSR had a dispute in the time frame about Bessarbia(?) - which the Romanians ceded under pressure as far as I recollect just pre-war. Again, from playing HOI rather than looking up the history. Keep it pre the T34 and it may be "interesting" (in the Chinese proverb sense..
!) to play as a short campaign as Romania in say 37/38.
Andy