First of all, I love the game, good job. Now for the suggestion:

This game format has the potential to have some very good mechanics added to it that would make it over the top. My idea is to have individual crusaders have level up choices. So for instance when you're looking at +100% damage, you should be looking at that choice or something comparable. The reason this would be great is because you can then make a higher variety of monsters to combat, and therefore make themed heroes with promotions that work to counter certain monsters. Obviously your choices would make you better against some types but worse against others.

Then on top of that you could phase your levels, maybe 50 level spreads that feature certain monster types more heavily than others. In this manner players would then want to level up their crusaders in such a way that they are designed to combat best in the level range they expect to make it to this playthrough.

Obviously as you get more and more buffs, being aligned perfectly for lower levels is unnecessary, but vital at the higher levels where you're pushing for a higher top level.

Here are some examples of monster types you could add:
Goliaths - Very slow, huge amounts of HP, huge damage
Swarmers - Many spawn at once, lower than average health, fast speed, lower than average damage individually, but in large numbers kill fast
Support - Very slow moving, stop at mid field, give a variety of buffs i.e. haste aura, healing aura, defense aura, attack damage aura
Ranged - Stops around mid field and targets heroes from afar.
Mages - Stops in the far backfield, targets heroes from afar with aoe damage.

Then you combat these monster types these are some promotions you could make available as alternate choices to just the bonus damage selections
Champion - Large bonus dps to Goliaths and Bosses
Magic Power - Do a percentage of your damage to all enemies
Support Assassin - Targets supports first and deal bonus damage when damaging them
Ranged Killer - Targets Ranged first and deals bonus damage when damaging them
Mage Slayer - Targets Mages first and deals bonus damage when damaging them

In addition, you could provide more choices for defensive and support themes
Regeneration for an individual
Greatly increase HP of crusader
Slow aura on the battlefield that makes enemies all take longer to reach you
Damage debuff aura so enemies deal less to your crusaders
Weakness debuff aura that makes enemies take more damage

Some crusaders you could even wipe out some of their damage options completely and just give them the choice between say ranged or mage counters. The last part that makes this system key is that the choices are semi permanent, as in the only way to swap them is to restart the world.

Boss fights - If you implement the above system with a wide variety of monster types and Crusader themes you could also make boss fights much more enjoyable because you can toss in all kinds of different monster spawning. For instance you could have a mage type boss with swarmers constantly spawning till you kill the boss, or you could have a goliath boss that has a bunch of support and ranged monsters that spawn. etc.