probably not. that would be broken. You'd have to completely change some fundamental aspects of the game to accommodate that. For instance, nullify would probably need to cost 2 or 3 more mana to reveal, as would every other mandatory enchantment. And a lot of the gameplay would be very different. Say you cast and reveal enfeeble on sardonyx. With a planning phase, if Sardonyx's controller didn't predict your enfeeble, they would have to wait until next round to dispel it. Without a planning phase, they could just pull it out of their spellbook and get rid of it instantly. There's probably plenty more examples of issues like that which could happen if there was no planning phase in arena.
