I just finished the campaign and was pretty disappointed.  The world was good - but it didn't feel like a Campaign, just a slightly modded game.  Here are a few things that I would do to improve it:

1) Main thing: Have a story unfold.  The FE Campaign did a good job with a story that progressed as you moved through the world.  I think it is totally possible to do something similar on a single map.  

a) Make part of the story forming a Kingdom alliance.  First have the Queen give some quests to prove your worth and loyalty.  Once creating the alliance with the Queen and learning about her kingdom, have her send the player on quests to unite the other kingdoms, learning about them and the world on the way.  Right now, you do not get to learn about the various kingdoms and empires - give us immersion. 

in those quests, make the player conquer various challenges - different types of enemies that require different tactics (great opportunity to teach about different weapon strengths).  Also have some quests the require gathering resources, or solving puzzles.  Maybe create levels that require certain units on specific tiles to progress to the next level, or enemies that can only be hurt by certain abilities/spells.  Something to mix it up a bit.

c) Once an alliance is formed with all the kingdoms, give quests that introduce us to the various empires.  Like with the trogs at the start, have them invade or pester the character in some way.  Maybe Kraxis converts one of your cities, but you don't want to put your citizens lives in danger, so you hatch an elaborate plan to sneak in from the sewers and take it back.  However it is done, let the player get to know their enemy.

2) Pull in more wildlands and make it mandatory that they be conquered.

3) Create a more rewarding ending... I finished and just thought, "That's it?"

4) Make armies more fun: 

a) I understand the logic of reducing exp in armies with lots of heros.  However, one hero per army gets kinda boring and a lot to manage.  Perhaps have "Hero slots" and "Unit Slots."  At the start of the game, have an army be able to contain one or two heros, and have techs be able to unlock additional hero spots in each army.  I want to be able to AT LEAST have a warrior and a healer/mage in my army without being penalized.

Create heros that evolve/grow over time.  It would be really cool to have a baby dragon hero near the start of the game that grows each time you find a special ruin or item.  Being able to decide what kind of dragon it becomes and what it's abilities are would be really fun!

5) Finally, create character growth.  Lord Relias is struggling with the loss of his lover and a war - give us insights into his psychology and thought process.  Get us involved in his story and make us want to see him win.  Make part of the story restoring his lover (or at least trying) and almost risking the entire fate of the kingdoms on that goal.  It would be so cool to see that story line build and could be an AMAZING mid story point - Relias has finally found a way to restore his love, but doing so would put the Kingdoms at a huge loss (maybe it destroys all life shards or have there be a magic barrier that has been holding the empire hoards back and have that fall and a full scale war begin).

Overall - make it a STORY and make it EPIC.