I propose that we should limit the amount of heroes that can be deployed in a battle to 8 or 10. I'll give my reasons for this:
1. 8 heroes will give a maximum of 16 (17 with Avalon and no Bane) troops allowed to be deployed. For all practical purposes, this is plenty. I know you can deploy up to 18 troops if they are all 3x3 sized, but a full army of spiders, tanks, or anything else this size is not useful in any situation unless you use all tanks for a siege. Point being is, that 8 heroes will support 99% of troop setups that are used.
2. 8 heroes will allow ALL players, even those that don't pay, to have some sort of a chance in the endgame to be useful and able to have somewhat equal footing. Once you have 8 heroes, you can build them up the best you can.
3. This will remove the OVERLY used tactics of flooding a fight with all of your heroes to make sure that you get your 30% bonuses (which is too much anyway, imo). By removing this tactic, you will have a much more reasonable spread of power and the bonuses will be much more in line with how I think it was intended.
4. MOST IMPORTANTLY, I think, having only 8 heroes would promote players to use more troops, which comes with more strategy, as well as find a reason to use super units and dragons in the fight. With everyone (on my server at least) using like, 16 heroes or whatever, the only tactic is to put 2 or 3 lines of troops in the front, load the back with heroes, then pray you have better hero stats than the other guy. Also, you would take in only the heroes that you feel would be useful in the fight with their abilities. For example, I've seen people putting might gear on mages, just so they can boost their might and get the 30% attack bonus because they know that they won't be using those abilities in the fight. With only 8 heroes, you would want to play towards the strengths of your heroes because you will be much more likely to use all of your abilities.
I hope I've made a good enough argument for this. On a side note, even limiting to 10 heroes would be good.
Lastly, one negative about this that might come up, is everyone using the same 8 heroes. I don't really think this is too much of a case, but that would be something that the devs would need to work on balancing.