True, Spain can be an option. Put one INF in Spain and all the tanks you can muster in GIB. This way, Germany can’t attack your armor (unless they have a strong Luftwaffe), but you can blitz to WEU next round combined with an amph assault in Normandy. Should Germany decide to take Spain, no problem. You move in your armor and all the INF you can using your trannies. Should Germany try and attack you after this, you get the benefit of using your INF for defense instead of offense. All in all, you’re better off using two rounds of unloading and then defending than attacking with one round of transports. You can of course also put an AA in there alongside the rest of it.
As for Mongolia, it has a small advantage; namely that you can threaten both Moscow and India at the same time (not directly of course). Often, as Japan I go for Moscow first, keeping Mongolia, China and Sinkiang, retaking French Indochina every so often, not even worrying about India unless the opportunity presents itself. From Mongolia, this is easier.
Besides from these two, I could imagine Turkey playing a part as a relay for bombers strafing the Med if Caucasus is unavailable to the Allies. Needless to say, this needs INF backup.