A raise in the level cap would be silly. If the rate at which you get powers continues, everyone would have four additional powers. Maybe one would be nice, but after that? I'd probably pick up powers that aren't really helping me at all. I can't imagine that I, or anyone else, could make a character that really needed those extra powers. Even then, there's all the additional slots you'd be getting. I think the number of slots as it was, was perfect. Yeah, sometimes I wish I had an extra slot or two, but I always made due. With that many additional levels, I'd have excess slots. And that just doesn't sound cool. This game doesn't need to be any easier than it already was.
Also, raising the level cap means creating brand new content for 50-60. Sure, I guess you could just make all the existing level 50 content scale up to 60, but that just doesn't...feel right, or something. Also, IOs scale with levels, and level 60 IOs would be insane. Base level 60 IOs granting more of a percentage than pre-I3 HOs, purple sets that only enhance one boost (like damage) giving 75%! That doesn't even include Enhancement Boosters. Sure, there's ED, but with this much of a percentage increase, characters will be becoming vastly more powerful. Add on top of this having extra powers and slots. No thanks.
Speaking of IOs, if I were to, say, get my character(s) back as a level 50, fully decked out in IOs and with +5 Enhancement Boosters, the disappoint of knowing all those level 50 enhancements are useless because there are newer, better level 60 ones would just be too much to bear.