Yeah, the respecs aren't immediately necessary... but people are excited and want to get there. By being careful about where you get slots from, or by simply choosing new powers that don't need many slots, a character's performance should absolutely not be going down... yeah, a power or two might go from say six slotted to five slotted, but an entirely new power will be more than compensating for that. I know that most of my characters will be getting powers that I've always wished they could have, but couldn't fit in my build.
And you already can have a fourth tray. And a fifth, sixth, seventh, etc.
As for Mids', there's the unofficial patches out there, but if you're only concerned with figuring out how many slots to put in which power at which time, an easy solution is to put your Stamina slots in say, Rest.