I'll keep it realistic with three simple things I haven't seen mentioned already:
1. Bring *back* the ability to assign specific terrains to a kingdom or empire. I believe it was only WoM where each player/opponent had a unique terrain/biome attached to them, and would spread within all settlements; not just limited to grassland and "evil" wasteland. You could mod/assign your custom kingdoms/empires to have all their settlements spread deserts, snow, barren/rock, and I think one more. You could make an empire spread grassland if you wanted to, for example. And now, I think a few additional biome types should be included as well, like swamp, jungle, fire/lava area, and perhaps a few newer ones I'm forgetting.
I know why this feature was removed, but between the advancements in Elemental and the machines we can run it on, I'd really, *really* like to see this finally come back to LH. The unique feel this gives to each kingdom is irreplaceable IMO.
2. Make outposts more complex and interesting. I haven't been playing for awhile, but I don't believe this has been addressed. Either make them harder to build, or use another method to keep the AI from spamming them so much. I think there should be better ways to defend them, like being able to train guards at the outposts, and then they must stay there. Have different types of outposts -- perhaps an "elemental outpost" that can slightly amplify the power of nearby shards, and it can train mages. There is a lot of potential here, but outposts in their current state are missing something. To implement this is probably the most involved of my three points, but I believe it's needed.
3. More random events, that actually feel random. The same events seem to happen to me in every game, and in the same order. I know every single time I hit mid-game, like clockwork, every opponent will suddenly go insane/enraged and all start attacking each other as a "random event". This completely kills replayability for me on some level. Just adding some new events for variety sake, and not making the time the events occur in such a predictable manner will strengthen LH immensely.
I'd also love bringing back naval exploration, making it decent and incorporating naval battles. I'd love bringing back dynasties, raising your child by making various decisions about them that craft them into who they will be as an adult. I'd love being able to capture important units, and be able to "trade" them back for rewards like specific items or gold, or perhaps your own units that your opponent has captured and held for ransom. I'd love detailed, hand-crafted cities that you can put great detail into, including your own castle and throne room. Your sovereign can freely walk around in settlements/cities, talk to people to get quests, hear townsfolk opinions on you and the world around them, do training, shop for items, catch spies, get into fights with rebels, assassins, drunks, etc. I'd love a full-fledged dungeon system from traditional caves/ruins to forest and mountain areas with multiple areas where you can move as you frequently encounter enemies and loot. I don't know what kind of hybrid game this is anymore, but dammit, doesn't it sound like the most immersive and wonderful game you've ever played? Ok, this is obviously just an incredibly-lofty wishlist dreamlist for the next game, so I'll stop now.