With a "true 4X" game my goal would be to make a turn-based game that moves quickly, has a lot of interesting decisions, and isn't just a slug-fest in terms of combat (since that's one of the places that turns in multiplayer in particular really tend to bog down).
One solution from a non-multi-player perspective is the one animals use in the wild. Only when 2 even matched animals meet is there a true battle. Otherwise there is just a lot of posturing. That is the human would have to convince the AI that victory is impossible and it will resign. This could put the emphasis/dependence on tech trees and knowledge of the enemy. For example, when Napoleon sent 1,000,000 troops into a Russian winter he still lost (even with overwhelming military dominance) but he might have won if he had better warmth tech. There are countless other examples, the US pulled out of Vietnam having never lost a battle, USSR lost badly in Afghanistan, the Ethiopians fought off Italy in WW2, etc.
In a multi-player game the resign feature would allow for removing the tedious, "I'm gonna get owned in an hour and half due to a slight but insurmountable deficiency in my army/base(s)." aspects of other RTS games (dawn of war) while introducing a fun bluff dynamic. This might not work with the online community but I have always appreciated that in a game of chess I could resign and play again quickly. Oh you got my queen, I resign, that was fun, lets go again. Further one could remove the resource cheap zurg rush if one lvl more advanced units where, say, 10X more capable. Send 1000 units against my 100 and you won't get slaughtered but you won't win either. Thus when to spend resources actually building an army/armada will be at least as important as building it.
This could also introduce a fun pokeresk "calling the bluff" aspect which no other game has. So, while two individuals may choose to engage in a cold war others may not. The possibilities seem limitless. For example, one could pretend to bluff, sucker their opponent into an untimely attack and then counter-attack.
If players could agree to a draw that would add another layer to the strategies implemented.