On your list of things to change, I see nothing that has anything to do with affecting arena size,only things you wish were different? When I look at your suggestions I see most of them would be detrimental to game play, and have 0 impact on why split arenas were put in place.
HiTech
Split arenas were put in place to spread players out and encourage a more equitable distribution of force...What better way to spread them out without having to restrict their ability to play together in the same environment than to diversify the relevance and importance of additional and existing infrastructure.
For example, take my two examples of base size and type specialization and factories. Imagine if we had a more meaningful, less hap-hazard and random, distribution of field sizes so that in every 3-5 square sector area there was only one large field, which was also the zone field and the only field capable of launching 4 engine bombers supported by vbases and med/small fields in a loose concentric circle. Then say we have factories that respectively represent production of German/Italian planes, British Planes, US planes, Jap planes, Jets/Rockets, Prop Perk planes, Etc.
Now, imagine the the map living in real-time. One force is attacking your German aircraft factory near the front, another force is attacking your only large field near their British aircraft factory after capturing some of its adjacent supporting fields, while yet another is attacking your only port in another area that is the supply point for two friendly task groups in the vicinity conducting offensive operations. If the port is lost, the task groups will cease to regenerate battle damage..Of course you would also be doing similar things to them elsewhere and likewise the third country.
What would that do to the distribution of force in the MA? It would spread them out, it would rally different subsets of players to different objectives at different times for different reasons, which in and of itself, as an ancillary benefit, would help unify the community. For example, if your German Aircraft factory is under attack, every flier who loves Luftwaffe will rise up like hornets in defense. Similarly, the port being under attack will cause those who like amphibious and seaborne sneak attacks to get involved. The large zone field under attack will cause the "strategists" and the bomber guys to get their freak on...etc...
Suddenly, a map that was once the monotonous repetition of the exact same handful of activities for the exact same reason, in the exact same way, over and over becomes a diverse and intriguing playing surface with a wide variety of strategic considerations and reasons to distribute force in a focused way, but over a plethora of single contentious focal points rather than just 1 or 2. As it is currently on the HUGe maps, there's no compelling reason to really defend anything, so most just look for the biggest fight on the map and just pile on, or gather an overwhelming force to steamroll the vast expanses of undefended portions of the map. The design itself is largely to blame for the problem you are trying to cure with the arena caps, it would be far more logical and rewarding to simply use some imagination and vision to tweak your design.
If you can't see how creative solutions along these lines would be far more interesting and exciting, while at the same time achieving the gameplay goals while pleasing rather than disgruntling your customers, I don't know what else I can tell you...It's as obvious as a dog's balls to me...