First, I'll say again how much I enjoy this mod. I thought I had "conquered all the worlds" HOI2 had to offer before trying this mod. The greater challenge yet maintenance of historical accuracy and flavor is excellent. Also, the campaign still progresses in new ways each time you play it. Just when I seem to have developed a strategy to defeat the Soviets in '43 (in Bring 'em On), the AI seems to be reacting differently in the next game....
As for improvements, I really can only think of a few minor ones:
1.) Instead of the airborne raid events, perhaps it would be better to just give the UK 3 paras and 3 transports. Then they can use them in coordination with an invasion? I don't like that the events give the provinces to the UK even if you have an army defending it which defeats the invaders. They should still have to win to get the province. Plus it would make more sense for them to attempt the raids in coordination with an invasion, rather than fairly useless forays.
2.) Can anything be done to prevent the US from invading China? I don't think it takes too much away from the Europe, as I haven't seen more than a few divisions there, but it just looks bizarre. In my current game, Japan has taken Midway while the US has taken Korea. It just doesn't make sense. I wonder if it would be worth giving Japan the Philippines by event, to force the US into a more realistic island hopping campaign. Or perhaps there is a way to block US access to China until after they have taken other key provinces. Such changes would make it too easy for a human player, but I suspect that the Jap AI would not exploit them too terribly....Asia is irrelevant to most of the game until around '45 or '46. But then it can be somewhat significant if you haven't taken the UK yet, as I usually don't until '45 in TRM.
3.) If you are going to update the map, I would vote for the Darkest Hour one. Changing it will take a lot of time and effort to rebalance it as well as it is now, though.
4.) Have you considered making TRM-like mods for other countries? I almost always play Germany because it is the only major power that offers a real challenge (I don't really consider Japan a major power). US, UK, and Soviet games are just too easy because the AI just can't effectively manage the German military effort (which says something about what the German military accomplished despite tremendous disadvantages as well as how unprepared their opponents were at the start). I somehow doubt anything could be done with the US or USSR unless you started it around '41, but if you diminished the US and made a strong Germany, might be able to make a fun game for the UK. It would be nice to be able to play a challenging game on the Allied side, too.
Again, great mod. I have spent more hours than I care to admit on this game, and this is the most fun mod I have played.