i have some screenshots ill post later, but i have some ideas/suggestions:
1. make mordor have more orcs spawn (not more often, just more) and the same goes for isengard with uruk-hai. also, i realize that i have never had to even remotely defend helms deep, nor have i lost edoras. and ive been playing the default Illuvatar with most aggressive set for the AI. My idea here is why not put 2 resources on teh map, one under edoras and one under helms deep that are required for isengard to build some new units. First off, i think the unit that comes from edoras shouldnt really be that great... maybe even for them to have those gunpowder things in the movies that could act like cruise missiles. but for helms deep they should be able to use that resourse to make some sort of seige weapon, say a cannon, or something that didnt really exist but could conceivably follow from what saruman was doing that is massively powerful. anyway the point of all this is that it would give the AI a huge incentive to to attack Rohan since they currently dont... at all. Also, maybe do a similar thing for Minas Tirith... i dunno, but i dont think they attack that city with much force.
My second idea is to give Isengard a small wonder called "Grima Wormtongue" or something thereabouts that acts like the intelligence agency in vanilla, but make the cost of planting spies astronomical so it wont be done. however, give them a spy in gondor/rohan from the start so they can spy on cities, incite revolution, etc. basically this is just to help/strengthen the vastly underpowered isengard.
also, the undead spawn too late. i usually finish around turn 38-40 if i try to do the Ring victory so the turn 48 undead dont really help.
in addition, the actual area of Mordor is far too empty and easy to invade. By the time ellasar and 4-5 armies of rohirrim are at the gates of minas morgul, mordor has nothing left. at the very least i think there should be a lot more defenders/orcs/units running around inside of the mountains around mordor. I mean aragorn would never have been able to defeat the army that mordor still had when he was at the black gate... they would have been completely overwhelmed. now i still think the conquest should be possible, but it should be MUCH harder then it currently is.
and my final thought is that aside from the difficulty of not losing any hero units (other than boromir) this scenario is too easy! i mean dont get me wrong i absolutely love it, but i want hordes of orcs smashing against my walls and i want to be forced to abandon osgiliath and edoras and i want to fight an epic battle every once in a while to defend a key position/city.
these are all suggestions, but keep up the fantastic work.