I shall preface this with an opinion - I believe shoulder charge into guard-break is overpowered and in serious need of outright replacement. I don't think it promotes skilled play on either end of the maneuver and I don't think even tweaking it will help. At lower levels, this is a death sentence for even the most determined, and at higher levels turns every chain into a guessing game with no right answer. The only 'counter' is to roll backwards, if there is even room to, if you have stamina to, and does nothing to prevent him from restarting the chain again once he's closed the distance.
I propose a replacement and welcome anyone else to do the same.
1) Do away with shoulder-charge entirely. By itself, it's fairly useless. Any competent player will dodge it, and may even punish it. It's only the mix-up that gives it real strength outside low-level play, where people simply don't know what to do.
2) Instead of allowing the shoulder-charge to be used on its own (after a dash) or after an attack (just a light), put it as a part of a special chain.
Crushing Counter into Guard-break activates the shoulder-charge, allowing a light attack to immediately follow. This will prevent chain-spamming the mixup, will offer a counter (don't attack top if unsafe), while rewarding Wardens with sharp reflexes rather than those that can just spam the mixup again and again.
Now, I know plenty will want to argue that this combo in full would deal immense damage. You're right, entirely. But it doesn't compare to what I've seen done with the mix-up against an opponent that is simply forced to flip a coin and can't catch a break. I've been on the receiving end and been the one to bully people with it. It isn't fun, at all. I've stopped using it entirely because winning with it just leaves a terrible taste in my mouth and spoils any sense of fun this otherwise fantastic game provides.
My experience - Borderline Rep 1 Warden, mained the Warden throughout both Betas and all of Release, and have spent an immense amount of time in Practice solely to examine the mixup itself.