Is this something people might want to see as part of a difficulty mod?
No, because while that might seem like a small change, I can virtually guarantee that it would have some massive effects on the balance of the game. You'd have to do a lot more work than you're expecting.
Oh well what's the fun of a rebalance/difficulty mod that doesn't push the boundaries :P I don't think the game is going to be suddenly completely different if you knockdown your allies while battering ram; you'll just have to be more careful about positioning. Most of the time you'll just be able to take a couple steps over, but if you aren't thinking, you might find yourself wanting to battering ram through two teammates because repositioning would make you eat an attack of opportunity. Some wide-ranging AoEs like Mass Disease would probably be hard to use, so maybe abilities like that wouldn't get affected, though then again Mass Disease is kind of OP from what I hear anyway.
It could have the general effect that melee characters are put at a disadvantage because mages and rangers will have less control over their abilities and so will generally want melee characters out of the way. But most damaging abilities already cause friendly fire or are single target or small aoe though. Really, it's actually not that many abilities that would be affected. At most: Earthquake, Mass Disease, Mass Slow, Horrific Scream, Splintered Arrow, Arrow Spray, Rain of Arrows, all special arrows, Battering Ram, Whirlwind, Divine Light. Half of those are master abilities that, as mentioned, people often use as openers anyway or are overpowered as is. I mean, I could also make buffs (like mass healing) apply to enemies as well, but that's probably too much.
I do think giving AI more friendly fire could have more unforseen consequences than player FF, but I'm not quite sure how many enemies use AoE. Probably just another thing for them to stumble over.