I concede that the coin-toss cleric is a bad idea. At least on paper not having playtested this. However maybe Minotaur (or any ECL-monster) can take that spot. I agree that the rocket tag style should be avoided so I don't think it's a good idea to balance a low rate of success with a high rate of effect.
Yeah, there's definitely room for non-standard classes. I'm not saying that folks shouldn't be allowed to play "Imperial Guard" if they want to, just that it should be obviously called out as a choice that is off the beaten path, and that may not be for everyone.
–No, the balancing should be made with exclusive class features instead. That is if you want to be able to find traps, or rage, you need to be a rogue or barbarian. A class is a complete package that offers a unique play experience. I know we have been taught not to balance a crunch bonus with a fluff bonus but having seen the result of the perfectly - crunchwise - balanced D&D I'm not so sure anymore.
Eh, I'm not so sure about this. I have no issues with rage being a barbarian ability, but I think anyone (with the appropriate skill) should be able to find traps. Rage is not a fundamental element of the game, whereas finding traps is.
I still recall the aggravation it caused me (in earlier editions), when no one chose to play a rogue. On the one hand, I didn't want to punish the players for playing what they wanted to play. On the other hand, traps became far more punishing without a rogue, and I had to use them much more sparingly as a result (because I felt that to do otherwise would be unfair). I hated that, and would not like to see a return to it.
I have no issues with a particular class making certain types of challenges
easier. However, in my opinion, class abilities that trivialize encounters (Turn Undead), or where the lack thereof makes certain types of encounters unapproachable (Find/Remove Traps), should not exist.
Note that certain play styles circumvented such limitations by role playing through trap encounters. However, I played with some DMs who wouldn't give hints about the location of a trap. It can be nigh impossible to role play your way around a trap that you have no idea exists. As such, if non-rogue parties are expected to role-play their way, that should be explicitly stated in the rules.
About the cleric though ...and watch me grasp at straws. Clerics have lots of class features that are unavailable to the other (four core) classes. Clerics can heal, cast spells, fight in melee, turn undead, know religion, use domain powers, and so on. The breadth of exclusive abilities might be worth a middling chance of success. There is even room to give them 60% which is about norm nowadays anyway.
Yeah, there's room for variation. I just think it should fall within 55-75% success for class abilities. If your 3 charisma Fighter wants to train in the Diplomacy, and still only has a 30% chance to succeed, well... he's got a 3 charisma! However, a fighter shouldn't have a 30% chance to successfully hit with his sword (unless perhaps he's suffering some terrible circumstances, such as in the example above.
Obviously, even if you follow that rule, you can make exceptions. I just think that those exceptions should be made for good reasons (such as designing a Gambler class whose entire shtick is built upon the principle of swinginess) and not simply for the sake of superficial differences between classes.