Oh well, I modded my game to include Iron deposits in Syria and Smolensk, so any faction could get access to Iron rich provinces without going to great lengths in conquering.

I also think Asia should be revisited, perhaps with focus on China and India.

Map edges are indeed easy to defend, but there are few, most can be accessed by shipping. Only the players have the strategic intelligence to continually guard their coastline, that's where most of the easiness feel comes from, AI factions can be invaded from the sea, just need a little planning. The Sahara thought is very interesting, but possible hard to mod, I don't know...also it would affect adjacent faction much more than those living in secure regions.

Perhaps, but I think missile fire out of a forest is still doing its job, especially due to the morale malus it inflicts upon the enemy troops.

Yes, there are lots of ways to upgrade diplomacy, in fact it is already better compared to Shogun, and I hope there will be another progress in Rome, eg Military alliances, movement through allied territory, proposals from you or by AI players to attack a certain province together...
And about the superpower, did it never happen to you that everybody was at war with you once your empire was the biggest? Same normally happens when an AI empire gets too big. They would be lucky to have even one ally by then...