By Epic Rampage, do you mean Epic Rage? I'm not sure I'd go so far as to say that is a broken power. The drop to D3 for that turn is a major effect. Generally you will only boost yourself in cases where you're facing an opponent with high Defense. Most of those also have good Strength. Since combat (for most situations) is simultaneous you will be scoring some extra Wounds for sure with the +4 Strength, but you will find yourself getting back almost as good as you give. Having played Wood Elves in WotR a bit I can tell you that being on the receiving end of a combat with Uruks, Morannon Orc, etc. when you're D3 is NOT a pleasant experience.
I'd say that your Rage against a high-D enemy, and them hitting back against your D3, is probably going to end with about as many models lost on each side, all other things being equal.
Now, not everything NEEDS to be equal of course. I don't mind Galadriel using Epic Rage if she's with a formation of Wood Elves (no negative impact). And if you have Epic Rage in a cavalry force charging infantry (strike first) then that's also going to tip the scales. Or if you have another Hero that can call Epic Defense (but that burns another Might).
After the whole discussion of the "danger" of Epic Strike and having played a few games with it used by and against me I'm pretty much settling down that Epic actions are NOT a major unbalance factor in most conditions (you probably have to cheeze your list to do so). Most EH only have 3 Might and some combos will take 2 Might to pull off, so you may have it happen once in a game.
Magic is really where I think the greatest area for abuse and the greatest risk of deviation from Tolkien's ME flavor happens with WotR. That's where I think the best benefit for change comes. Get rid of most generic spell casters, or limit them to a subset of spells rather than whole lists or limit how many times they can cast in a game (having 3 Stormcallers blasting Nature's Wrath repeatedly is ugly). Make Nazgul a little more expensive across the board (try increases of +25 to +50 to find the sweet spot). Those two changes alone would have a positive impact, but just a start.