I echo Durk's post. I am also a solo player, and have played all the AGEOD titles released at least once this way. I like to keep as close to the historical events as possible, but at the same time allow some flexibility to pursue actions that a nation/faction could have taken within the realms of reality. I am currently playing a massive solo 1850 campaign in PON taking control of some 20+ nations.
In undertaking this massive campaign, I wanted to achieve several things. The list of nations that I currently control is as follows:
United States of America, Ottoman Empire, Spain, Russia, Austria, Piedmont-Sardinia, Belgium, Prussia, Japan, Portugal, the Netherlands, Great Britain, France, China, Egypt, Mexico, Taiping, Persia, Brazil, Argentina, Chile, Peru, Bolivia, Paraguay and Colombia
Obviously choosing the major nations to play is a given. I also chose to control those nations that have or will acquire colonies. Then I added much of South America to my playing list, and finally some of the other nations that featured more prominently during this historical period. This has allowed me to pursue a relatively historical path, plus ensure the world economy functions better by developing some of those nations that the raw materials the more developed ones need (i.e. South America). Additionally, I have been able to experiment and change my playing style as required. For instance, rather than trying to make a nation self-sufficient in all resources possible, I realised it is better to develop those that a nation is strong in and trade for those that it is deficient in, thus creating a more natural economy/trade system. It's also been fun developing the economies of the 'minor' nations; Brazil is a potential powerhouse once there is enough investment available.
PON is, IMHO, the best platform for solo play in that the research and development structure ensures almost every faction has differing levels of capability. Rather than facing enemies with similar or same weaponry and force structure, due to the research tree in PON I have nations at different levels facing off against each other. For instance, In my campaign Persia has developed muzzle-loaded rifles well before Prussia and Austria have. Conversely, China and Japan are a generation behind having just researched percussion cap muskets.
Controlling so many nations means I often forget what moves I have made with one when controlling an opposing side. Additionally, the sheer number of rival nations present in PON allows a great variety of potential conflicts. In choosing South American nations I also get to model some of the conflicts that occurred on that continent during the period (i.e. War of the Pacific, War of the Triple Alliance). For example, Argentina is an unstable, volatile nation in the 1850's and I decided that as a distraction from domestic woes, they would attack their equally unstable neighbour in Uruguay. So far the war is not going to plan for the Argentines, with the added complication of both Brazil and Paraguay waiting in the winds deciding if they should intervene on one side or the other. Taking control of some of the other factions also allows me to replicate some of the big conflicts of the time. My current Taiping Rebellion has so far lasted three years and resulted in the deaths of over a million soldiers on both sides. In controlling the conflict, I also get to experience the events written into the game. Rather than the rebellion being easily crushed or in constant stalemate due to AI control, I will actually get to see the arrival of the Ever Victorious Army, 'Chinese' Gordon, and the massive death and destruction the Taiping-Qing conflict caused. Interestingly, in my game there has not been a Crimean War - so complete historical simulation is not guaranteed!
It might be taking forever to play, but it is fun doing so!!