You forget that people will be doing Naxx in around 200ilvl gear. Also, people are absolutely clueless when it comes to Naxx. Very few guilds will be able to clear it at first even as it is right now. Blizzard's main mistake with old Naxx was how hard they made it and how late they released it, resulting in VERY few guilds having cleared it at all. Don't do the same mistake. The raid will become more frustrating than actually challenging and fun. My vote is a hard "NO".
Of course, if a guild knows what it's doing, then Naxx will not be a challenge for them, but assuming that most of the guilds will know Naxx in case your WotLK realms all have opened and unscripted FoS+PoS, ToC, OS etc. etc. free EoTs, free T9 etc. etc. without ever attempting T7 & T8 content, and will be geared for Naxx when attempting it is silly. If you want hard raids, make sure Ulduar will be 101% scripted. Leave Naxx as it is.