Something along the lines of a more unique PvE wouldn't be that major a queue cost as to warrant concern, since fundamentally such a mode would cover almost none of why people actually wish to play League.

The most significant cost here is development time. Game engineers are a limited resource since we're very picky about the engineers we hire. Right now, I don't think that's a suitable cost for such a mode.

Though, that's also why we have thunderdomes. So, who knows?