If I were writing the rules (and I haven't been following the 5e posts enough to know how close this is) and getting rid of skill roles as something independent from ability checks, what I would do is something like this: different classes get skill bonuses relating to their domains. For example: the rogue gets +3 on skill checks relating to traps, regardless of if its a wisdom based "spot" check, a dexterity based "disable" check, etc. The mage would get a +3 bonus for arcana related checks, regardless of if he is Identifying a spell, detecting magic, etc. The dumb as rocks barbarian adventurer can probably figure out that the pointy hat guy in the corner yelling "Magic Missile" over and over is probably casting magic missile, or that the glowing shimmering curtain of light is a magical effect even if if he isn't trained in arcana.
I'd try and adjust the bonus numbers so that the play experience works about like what you'd expect; a dexterous fighter could sometimes disable a trap, but you wouldn't want to count on it, a rogue could dependably do it, and a low dex guy would almost never succeed.
I'm not a fan of trained only skill classes because IME it leads to forcing the players to have that specific class feature on hand, which means either giving that feature to almost every class or forcing someone into a class they don't really want.