I had an idea a while ago that I forgot to share...
I was thinking, since in RoC the difficulty level is truly set by your civ choice more than the difficulty level, perhaps this could be made clearer in the civ selection screen. Since that screen's background can be edited per mod, you can color code the background area around the name of each civ by difficulty, with a little legend somewhere near the selection box. You could have five difficulty levels, represented (from easiest to hardest) by dark green, light green, yellow, orange, and red. That way you could make sure a player knows what they're getting into. And then you can change the difficulty levels so the lower and higher levels are a bit closer to the supported levels in hope of making the difficulty level of the mod a bit closer to that of the regular game. (High level tech costs don't have to necessarily be slashed that much, there are other ways to increase difficulty.)