I want more side characters, I love the side characters, and I think that there could have been some really fantastic exploration of Sam and Dean through side characters and still can be - exploration of Dean through Cas and exploration of Sam through Kevin, that sort of thing - but that shouldn't be a replacement for exploring Sam and Dean in light of each other.
I don't think development can be an all or nothing thing, but I think fresh blood could give Dean and Sam the impetus to have their relationship grow a bit, as they each adjust to new circumstances, relationships, etc. Character growth shouldn't be entirely dependent on new relationships, of course, but they could carry back to Sam and Dean in interesting ways/help give the brothers a story that grows them both as individuals and in relation to each other. (It's not the only way to do it, but after S7, I think it would be one of the easier ways).
Re: Extended TV Guide spoilers
I want more side characters, I love the side characters, and I think that there could have been some really fantastic exploration of Sam and Dean through side characters and still can be - exploration of Dean through Cas and exploration of Sam through Kevin, that sort of thing - but that shouldn't be a replacement for exploring Sam and Dean in light of each other.
I don't think development can be an all or nothing thing, but I think fresh blood could give Dean and Sam the impetus to have their relationship grow a bit, as they each adjust to new circumstances, relationships, etc. Character growth shouldn't be entirely dependent on new relationships, of course, but they could carry back to Sam and Dean in interesting ways/help give the brothers a story that grows them both as individuals and in relation to each other. (It's not the only way to do it, but after S7, I think it would be one of the easier ways).