We already saw it with T5. Pre nerf SSC/TK absolutely decimated dad guilds. It was like Naxx40 but with no world buffs to brute force the content.
I think Wrath will be fine as is. T7 and T9 can be your easy tiers (T7 even has sarth 3d which will be difficult for casuals). Those tiers can be made shorter in favor of longer Ulduar/ICC phases if needed.