wow, stop reading my thoughts, i was thinking exactly the same thing for the last few weeks, about the 50-150 f2.8.
personally, i don't see why people wouldn't love this one, it'd be the almost perfect 'all-purpose portrait lens', maybe 40-150 also possible. especially in a wedding or event, zoom to 150 to get fullbody shots of the happy couple from the back of the hall/church, out to 40 or 50 on a fullframe sensor for group shots. coupled with the 16-35 2.8 or just a 24/35 1.4 for the big group shots and you've got more versatility than the 24-70 + 70-200 2.8s (unless you have 2x FF bodies) imho.
150-300 would also be nice, but yeah, like everyone else has said it's gonna cost a packet, they've just gone and designed the 200-400 f4, and the 70-200s cover everything you can think of anyway.
i'm not in favour of making them efs only, frankly it doesn't make sense anyway, the only thing you need efs for is to get the rear element closer for ultra-wide angle shots. the only reason they made the efs 55-250 was to couple it with the 18-55 in dual lens kits (and make it lighter/cheaper).
personally, i'm still hanging out for the efs 30 1.4 as the 'aps-c nifty fifty' (maybe the 'dirty thirty'?)
and an ultrawide efs prime like the efs 15f2 or efs 11f2 (saw a patent rumour for one or the other here a month ago)