Specifically, I think it could be interesting to see more reaction to new enemy technology, such as how the Allies had to react to the appearance of heavier German tanks like the Panther and Tiger. This isn't something I experience often in games, but it happened twice while I was playing Endless Legend. The game was unusual, since it was my first game after the tutorial, and I took a large break of a few months between the first half or so of the game, and finishing it.
My break in play came just after/during the climax - I didn't feel I'd won the game yet when I stopped playing, but I realized I had fairly soon after picking it up again. Since it was my first game I hadn't been that aggressive, but was doing fairly well due to the low difficulty, and I'd fought off another nation and taken their capital. Playing as the Wild Walkers (sort of Wood Elves) my tactics relied on fighting in forests to get an attack boost, and hitting the enemy with masses of crossbow fire while covering my rangers behind tenai walkers (giant tree men). This worked excellently against the slow, tough soldiers of the Broken Lords I faced in my first war.
Fortunately, weapon upgrades in Endless Legend only require you to research a general technology allowing you to get better weapons of all types, with these improved weapons often requiring more rare materials. I already had a good weapon technology, so it was simple for me to design a new ranger variant using the other choice of ranged weapon - a longbow. While the crossbows are more effective up-close, cheaper, a tiny bit faster, and can be combined with a shield, the longbows have a higher attack value and deal more damage. But their real value is the Flying Slayer ability that comes with them. If I was smarter, given my tactics of keeping at distance, I should have switched to longbows already for the attack and damage boost, but better late than never.
The only challenge was building my new 'Necrohunters' fast enough to get a decent force together, and not using up all the uncommon materials they needed - but my existing forces managed to hold them off and soon my new units entered battle. The difference was day and night, with necrohunters in forests able to half kill a necrodrone unit with each attack. When they were being led by an experienced hero who could boost the range of all my archers up from four (already better than other faction units) to a massive five, necrohunters in forests just decimated necrodrone swarms, though getting caught in bad terrain could still leave us with casualties.
Things went much the same as before, with my longbow armed rangers massacring hordes of necrodrones, and my tactical successes easily countered their numerical advantage, allowing me to rapidly capture territory and expand my empire. This was when I knew I'd won. Nothing changed that state of affairs, but they did belatedly make a big change to their army makeup, switching from necrodrone groups to armies of centaurs - a smart choice since the centaurs are fast enough to run around my tenai guardians, but don't take massive damage from the Flying Slayer ability on my longbows. That led to some close battles, but we were too close then in productivity, and I could replace my losses.
I think the predictability of game technology is also to blame for the lack of moments like this - if I end up in a similar situation in future games of Endless Legend, I'm not going to have the same experience, instead probably more of a 'oh, time to make some units with weapon X' response. It makes me want to try a game mainly about researching new units and deciding which units to put your resources into, where you don't know how effective your new units will be until they've seen enough action - keep producing that new heavy tank which might get those reliability problems resolved soon, or switch focus back to the reliable but aging medium? Could be some interesting dilemmas.
*These examples are all air related since I've just finished reading the excellent Combat Crew by John Comer, which is an account by a B-17 gunner based in England from mid 1943 to early 1944.