Star Trek AH: no peace with the Klingons
OTL: The Praxis moon explosion forced the Klingons to make peace with the Federation.
ATL: The Klingons still try to fight on all their fronts despite the Praxis explosion damage, and make no peace. This strains the Empire so badly that it breaks up in a wave of internal rebellions. Compare the Austro-Hungarian Empire or Russia after 1918 but much worse. Add a quantity of planetary Kolokotronis'es (as in the Greek War of Independence) freeing their worlds of the leftovers of Klingon rule. The Federation has to cope with a risk of Klingon incursion, not an invasion but refugees. This spreads to the Ferengi Union, and the same rebellions and revenge by subject peoples who are no longer subjects. Planets try to rid themselves of Klingon cultural overlay, and of such wldlife nuisances as qagh worms let loose to breed everywhere. Etc.