In return, Sneak Attack, the Rogue class feature, increases her Sudden Strike damage to d8s and also triggers it while she flanks, it could even automatically deal half damage to crit-immunes.
So would a 1-level rogue dip do this? Or would it be better to add half your rogue level to sudden strike damage?
Hrm. Good point, and something for me to ponder.
I can see even more ways of casters geting yet more power. Now Invisibility is not only a powerfull defensive buff, it also provides extra damage to boot.
Are you honestly afraid of even a Trickery Cleric being able to sneak up behind someone via Invisibility and delivering +10d6 damage on a single attack?
And let's not talk about the monsters. Everything with multiple natural attacks just got even more deadlier. Against the poor dude in front that is. Wich will be probably a noncaster. Casters don't need to find ways to super buff their lesser minions anymore, just drop some invisibilities on them and watch the damage output jump trough the roof for minimal cost.
Unless the monsters with multiple natural attacks also have pounce or innate, constant invisibility, it's not nearly as huge a problem as you're making it sound. Summoned monsters, last time I checked, totally suck anyway, and spending 1 round to pull out a single, barely relevant creature, and then spending an additional 4th level spell slot to enable Sudden Strike for a few rounds doesn't seem to break the game in any way.
The casters have better uses of their time, and arguing that giving them a cute toy to play with that makes one of their shittiest options more playable also makes noncasters even more irrelevant doesn't exactly make sense. If this idea would somehow make Tier 3 casters Tier 2, or make Tier 3 warriors Tier 4s, or a combination of both, then, yes, your argument would make sense, but the idea does neither.
Meanwhile the rogue loses it's special trick for little gain. It already could sneak attack enemies normally immune to crits trough a variety of means, and increasing to 1d8 barely cuts it.
While this is a concern, it isn't exactly a huge one, at least not to me. Rogues are a Tier 4 class as it is, they could use some beefing up. With a little homebrew a Rogue would still be the king of backstabbing, but it could also become much more than that. The Rogue issue would definitely need to be dealt with though, yes, that is correct.
What do you call being caught flatfooted and being unable to add your Dex mod to AC then? That's what being caught suprised is about.
And yet, against creatures with Dex 10 or lower there is exactly 0% benefit/drawback. Uncanny Dodge also becomes a relevant class feature, rather than something no one cares about. It's something that competent and seasoned warriors should be getting, not thugs or the like.