If they fired the singer and got Johann back, then it would be epic. Loomis with the old Arch Enemy would be beyond epic...I daresay it would be better than a NM reunion.
I don't care how "metal" female singers may be in an extreme metal band, the music ALWAYS, ALWAYS, ALWAYS gets watered down in order to sell the hot singer chick image. Even if the music isn't watered down at first, it will be eventually as the record labels try to squeeze every last dime they can from what they consider to be the perfect cross of metal and sexuality in an ever-shrinking market. The band knows this as well and used Angela in the beginning to grab everyone's attention. I remember the promos for Wages of Sin with Angela's hot (at the time) body in a bikini without a head in an Arch Enemy advertisement in Metal Maniacs or some such mag. The band has always cashed in on Angela's looks and they're trying to do it with this new singer as well because they know that no matter how awful their album may be, it will still sell better with a hot female fronting the band than some old, sweaty, smelly dude.
Arch Enemy hasn't been a melodic death metal band since Burning Bridges. Since Wages of Sin they've been a metalcore band, and aside from a few decent tracks here and there, there's absolutely no reason to get excited about anything from Arch Enemy post-2001. There are a few moments of nostalgic epic-ness in Anthems of Rebellion, but they're fleeting. Khaos Legions has some great riffs but it quickly gets generic and Angela's voice is just grating after a while, even for death metal.
AE used to be a sort of diet Carcass, but now it's more like Faygo, juggalos and all. People don't realize just how important a singer's timbre is, that if he or she doesn't have that timbre to go with the music it's not going to work. Timbre and presence will outshine perfect notes any day of the week. Johann had that scathing, barking, rabid-sounding timbre in his voice that these two girls just don't have. Warrel isn't a perfect singer but he's a great singer because he has presence and a great timbre. People preferred Matt Barlow to Ripper Owens because of his timbre even though Owens can sing circles around Barlow. Same thing works for all styles of metal.