What you're asking for is very similar to the way city combat was done in Age of Wonders which I really enjoyed.  There, the combat maps were much larger, with a walled space in the middle and four gates leading out of it.  The only way attackers could get in at the units inside was by bashing down the gates or the walls with gates being easier to destroy, and both wall and gates had increased hitpoints if made out of stone instead of wood.  This mechanic actually made building walls in frontline cities quite useful, because a few well trained archers kill quite a few melee units that would have otherwise massacred them in the field.  I don't know how easy it would be to make something like that for Legendary Heroes and not make it feel tacked on, but if done properly I would pay for it.  The city combat from Age of Wonders is something I miss quite a bit in Legendary Heroes.

I suppose you could create a few new tactical maps for city combat with chokepoints.  You could populate those chokepoints with stationary barricade "units" that the defending player can switch from being impassable to passable.  This could let any defending attack the invaders but the invaders would have to destroy the barricades to be able to melee the units behind the barricades.  Catapults could do extra damage to said barricades.

Just placing barricades is not enough. These barricades will not protect from ranged attacks. In Age of Wonders there is an obstacle penalty while shooting through a fortification. There also is a limit for ranged attack. FELH doesn't have obstable system and limited range. So the DLC's task is to offers the solution in these conditions.

I think it's a hard task for modders. It needs making smart 3D fortifications, so the work is worth DLC and not just a mod.

Perhaps there is not much work for devs, and fans will buy this DLC at a good price. Don't devs need resources for the new GalCiv? Instead of preordering new GalCiv these may be real and quick money!

Lets count what should be done.

1. New 3D graphics for fortifications.

2. At least 1 combat map with preplaced "defensive" tiles.

3. Creation of "Gates" unit.

For basic concept there is nothing more to program. The project seems to be easy to be done. 3D designer is needed - and that's all.

Sounds good.  Yes i'd buy it.

Do you think it's too complex to make this DLC? But wouldn't most fans buy it if it costs not $5 but $10? Developers may easily get more funds to develop their next game - GalCiv. But if they are too busy with it and don't want to pay more attention to FELH, then at least we'll have what we have now. It's also not bad. But if there were sieges in FELH I think that the game would better compete with AoW3, which will arrive next year.

May be yes, but what if may be no? AoWSM (with mods) was the best MoM style game until FELH appeared, imho. I really like unit constructor and tech tree and graphics of FELH but AoWSM has its own bonuses, so if AoW3 were just AoWSM with 3d graphics I'd say that I'd had a tough choice. I really lack sieges. Naval is also needed. What adventure is without a ship?

Sieges, naval stuff..... and flying units.

That'd make a great expansion.  Where do I sign?

I'm ambivalent over siege warfare - I'd like for a city battle to be more distinct from a random fight in the wilderness (and if they could manage to make another distinction for wildlands...). But this is fantasy in the MoM tradition, and being forced to drag a siege train around is too "realistic" for the subgenre.

As for naval stuff, I say "no". There are two problems with even fully implementing naval transport - (1) "can you add naval combat in a DLC?", (2)  Total War: Empires was an expansion to a game that had ships, where naval combat/invasion was a primary mission goal - and look how badly they screwed up the AI! IMO, you have to bake in ships from the beginning, or save it for the expansion you finally totally revamp some of the core systems - you can't just layer it on.

I'm onboard with adding to the management subgame, though. I'd like to see some notion of the passage of time. I fondly remember the old-school board game "Blood Royale". Not as a game per se, from what little detail has lasted in my memory, the game system wasn't that spectacular, but it was a backdrop for personalities and dynasties that grew organically within each game session. So every time you came back, even with the same faction, it always played out differently and compellingly.

Any 4 sides is not an exploit in the current game mechanic since only one attackers army participate in combat. We don't ask to change the whole game mechanic, do we?

Also for current game mechanic - both naval and flying units are not possible. The is no possible settling on beaches and melee ground units without counterattacks can't do anything against flying units. Possibility to make flyers with bows makes most ground troops defenceless against them.

Simple siege wjould be enough. Just 1 new battlefield with defensive tiles on one side will be enough without need to change anyting.