Increase hero strength, most of them are useless, especially melee heroes
What? The heroes have saved me on several occasions. I raise them up to level 10 in just about every level after Icewind Pass and they dominate everything in front of them. Sure, at level 1 they're horrible, but that's why you train them! :P
A more complex upgrade path for towers and star upgrades. Like How you have two options for fourth version tower, you get two more more options for a fifth version per fourth version tower. Same thing for star upgrades. Also star upgrades for heroes, along with upgrades on level.