A successful defenders retreat, that is half a victory. The attacker has wasted a lot of resources and time to set up the decisive battle that would catch and crush the enemy, yet they are deprived of the glory and have even lost the initiative to the defender. But, and I cant stress this enough, the quintessence of a successful defenders retreat is to move your men and gear out of the mice trap before the attacker start the attack. If you wait too long it turn into defenders panic escape with huge casualties and loss of gear.
The way I see it is like, lets say Germany move a huge stack into Poland, and Russia have a stack in Eastern Poland, and it is obvious that Germany want to attack the Russian stack in next turn, then a successful defenders retreat will be if Russia non/combat move their stack into Belorussia in their turn, and leave a lone inf as blocker in Eastern Poland. This is already covered by the OOB rules. What you dudes want is the option to let Germany attack that Russian stack for a few rounds, and then suddenly the surviving Russians escape to an adjacent territory. It is extremely difficult for the defender to support and supply this kind of escape, usually all heavy equipment are lost. To keep it A&A style simple too is even more difficult, without getting too gamey where one step infantry can double move both in own and opponents turns. I figure the best is to just let it be. If the Russian inf stack can retreat back to Belo when Germany attack them, and then non combat move again in the Russian turn, that is not a punishment, its a gamey reward.
Naval battles on the other hand, should be treated different than land battles. Naval war is true maneuver warfare with all mobile units. Nothing would stop a defending ship from sailing away. But then it would not be A&A any more.
When it comes to attacker can retreat, I would change even that. If your assault get bugged down in the mud, your Tanks are out of gas and your men exhausted, its not like this units suddenly retreat to another territory just like a piece of cake,. man. In this case we should use the A&A 1914 rules with contested territories. The attacker in a land battle should never be able to retreat to another territory, but he could press to stop the attack and just contest the territory. Then the defender get the option to combat move out of that contested territory in his turn. The rule of thumb should IMHO be that you can only move your units during your turn. During other players turns your units are pretty much stuck. No more duble dipping.
But, like I said, naval warfare should be threatened different from land warfare, but that’s another topic
