Afforess, I think my options A and B could probably done as one option, the last is just because I'm a tad lazy. Another option I'd use is one to replace all the on-map unit graphics with counters like one might see in tabletop military strategy games, in order to beat down the memory requirements in the mid to late game (stupid MAF crashes...).
Randomness is right that there are a host of tweaks in RifE that may merit a look. As long as they're not too memory busting. I'd like to see the trade route limit bumped up to the point we can't surpass it (My coastal cities are right next to it towards the end of the industrial era, and I assume there are yet more things to give trade routes, but I'm also running Objectivism to keep my research going).
Civ 5? I'd do away with the food overflow generates population and instead grow city population at a rate depending primarily on the tiles controlled by the city and modified for surplus/deficit food, happiness, etc. Citizens would be determined by some formula of the population (say pop/10k, and take the square root). Then I'd take population out of the cities when building units and replacing casualties and use that effect as the basis for war weariness. I'd redo combat to something more like Panzer General (more unit stats, combat would rarely result in the outright destruction of a unit {as historically, losing an entire army was a pretty big deal} but frequently relocation to another tile or city, and I'd base first strikes on the relative range and/or mobility of the units). I'd change the upgrade from instantaneous effect to a process, basically I'd significantly increase the upkeep on an upgrading unit and give it an increasing probability each turn of completing the upgrade
I'd also calculate tech costs on the fly based on the global commerce output of the last 10 or 20 turns and how fast techs should go at that level of development, to get rid of all the 'this research takes forever' and the 'one research every two turns' turns. I'd separate technology from the applications (wonders, units, civics, etc,) and force choices to prioritize between applications. I'd accumulate all the GPP like great general points, empirewide, since I have so many cities that generate unused GPP (I'd probably include some effect to reward concentrations of a particular GPP type contribution) and I'd improve specialist options with technology (temples would allow priests, cathedrals would allow bishops who are worth an extra gold and GPP, the Apostolic Palace would give a free Pope who's even better).
Finally I'd throw out the old civ paradigm on trade being best at great distances. I think it's far more important (in the real world) to have trade going on within your nation than it is to have a handful of folks bring in sandwich toppings from the other side of the world. So I'd have each city attempt to send trade to every other city, with the strength of each effect based on the commerce at the home city and the distance to the destination based on the most efficient route (basically, multiply by the number of times I could cover that trip in, say, five turns, with a unit of movement mode appropriate to my tech level). Trade income would be the square or cube root of the total incoming trade effects. Each city would be a primary source of resources on tiles it controls (or produced by its buildings), and when trading to a city without such a resource, would increase the strength of its trade and make the destination city a secondary 'producer.' Cities with secondary resources could, depending on the resource and tech, similarly give them to tertiary producers. Production bonuses would then be given on builds the closer they are to each actual resource they need (though once researched, they'd always be buildable, just very slowly without those resources.
Of course, I'm just some guy with an opinion. I'm just saying there are changes to be made if they want to.
