Hello!
I really see some pack which buying very much players every month.
For example:
Power pack
But they can buy it only as pack in excatly day. if u are not in game u spend month and cann';t buy this.
I want propose create yet one purchase in game - subscription.
U simple sub on on pack and when u will want u can unsub from pack, very simple and no risk that u cann't buy pack if u are offline.