**there may be spoilers depending on where your at in the manga!! my theory will have spoilers for 421 so read only if you don't mind :)**
so tensa zangetsu told ichigo that if he used the final getsuga then he would loose his shinigami powers. and before he learned it, his dad told him that theres a reason they (they being the zanpakutos-being isshin's and ichigo's) don't want to teach this move. and we all know that way back in like episode 115, or close to there, when isshin killed 'grand fisher' he told uruhara-san that its good to have his powers back. and also he was talking to ishida's dad (whose name escapes right now) he said something about it being like 20 years. i promise i'll look it up tomorrow and make sure so bear with me for now. anyways this is my theory: isshin used the final getsuga and lost his shinigami powers but after a while he finally got them back, so why can't ichigo do the same thing? i know it'll take a while but theres still hope for him. :) i'm also hoping uruhara-san will have some way for ichigo to get his powers back sooner... idk these are just theories and wishful thinking... i was really sad cuz i thought maybe bleach may be ending soon since they just sealed away aizen and he lost his shinigami powers and what not. but then i was all "wait what about the royal guard and what not??" they haven't said anything about that dude so it like can't end!!! plus i've always had a hope really that ichigo's mom isn't dead. she's just in the royal guard or something...i know its just wishful thinking but whatever... plus the fourth movie coming out is supposed to be cannonz for like ichigo's master of full hollowfication or something or so i've read.
hope you enjoyed your theories! i'd like to hear yours too if you wanna share them. :)