Don't see it as "hey, don't play GT 5 anymore!"
it is never the case, we had some time ago people still playing the gt3, so gt 5 is still possible to play, it will just not have anything new, if most of the system basics logistic is keep on the gt6 it will be recomend to play gt5 so it wont be that much of shock.
Machine wise I am going to go in a similar direction IC² attempts to do right now, with electric/steam/other Motor Blocks attached to Machines to provide kinetic Energy for rotating stuff, and similar things, so that single-block electric Machines are only for cases where the Energy consumed happens to be electricity directly.
so we return to buildcraft machine system? both railcraft and forestry uses it , and most people arent that happy to use them.
in tthe end the only thing i realy dont like is the feeling of "not finished work", gt3, gt4 and gt5 now, simply stopped bein developed, no final release with major bug fix, no legacy.