Was thinking about the same thing today
Some thoughts on MP:
* Be good at super-fast early expansion - that's something you can easily practice in the single-player game. If you can keep up with the AI settlers in Emperor or Deity, I'm sure your coin will be worth something in MP...
* Pre-RR defence must be much better organized. I think we will be seeing garrisons of units placed so they can reach more than one city at a given time (by having them stay somewhere mid-between them - I've used this strategy in single-player, but it's quite superfluous), instead of being divided thin, each fortified in their cities.
After Amphibious Warfare, it's back to stacking defensive units into the cities; also you'll have to be careful what you keep near the coast. I think Marines will see much more usage in MP.
* Consider all the new dimensions in warfare.
For your invasions, expect a much stronger defence, with more clever use of terrain, more defensive lines and nasty fortifications. Expect walls in the Ancient Age.
For your defence, expect more bombard and more concentrated attackers. Expect more field fighting and long sieges for the first two ages. Your defence must have a strong force of attackers to beat invaders, not just "a set-up the AI will never be smart enough to overcome".
* Go through all the units and improvements in the game, especially those you think really suck. It will be a whole new world, and previously crap units are likely to be redeemed.
Compare for example the Longbowman and the Knight. The latter is universally preferred in single-player, but will that be true in MP? The Knight is almost twice as expensive to build, but has the same attack value. It also requires two strategic resources, while the Longbowman requires none. I expect hordes of Longbowmen being used alot in invasions (your stack of defenders and bombarders will be at move 1 anyway), while Knights will be critical in the field (which means that they will become more important to the defence than offence).
Keep in mind while considering the various units that cost will become much more important; your units will die more than in single-player, as your enemies won't let your 1-hp elites escape without a fight. Also resource requirements are important, since you'll find yourself much more cut-off from your resources than you're used to - we're all getting quite skilled at resource-denial, and will use it mercilessly.
In single-player we're always thinking about upgrade-paths. The importance of upgrading will diminish in MP, I think, as units will have to be reproduced alot. This alone calls for a re-evalution of the unit list.
* Re-consider also the different civs and their abilities. Actually I think the popular traits will become even more popular, particularly industriousness. Wouldn't want to be without that one in MP. UU's will be very important. Some of them will be much more useful - War Elephants come to mind.
Okay, those were some ideas I just had from the top of my mind... will be reading this thread and adding more comments. Very interesting subject!
