Since you’re just shopping for you now… you should pick out panties you absolutely love :-) But if you crave the humiliation… then ask the salesgirls opinions on size, elasticity, and proper care of delicates :-)
This is a fun one… communication is key sweety. You need to start that road simply by having the talk about BOTH of yours sexual fantasies (and don’t let her cop out… women do have them). You have to steer the conversation all to her wants and desires and your utmost desire to fulfill all of her fantasies. It shouldn’t be about you (submissive-remember?). Once you’re both comfortable talking about all those naughty things she’s dreamed about but never told before… that’s when you can make your point of how you want to serve all of those fantasies (and serve her).
The cuckolding thing is a bit harder… But if you laid the right groundwork with her fantasies… her dominant side will get more comfortable and you can slowly introduce the idea of sharing or watching her. You have to be careful and really talk about it so she doesn’t think it’s just a ploy for you to be able to play with other women though. You can start those discussions slowly with porn (yes… women like porn too). Threesome, voyeur and even gangbang porn where the women’s joy is evident. Don’t pick masochistic or degrading stuff to start with. Judge her reactions to it… or better yet… go down on her and make her really happy while she watches (again… submissive). Soon enough you’ll be able to talk about how you’re ok with her doing those things and how you would enjoy it too.
Main things… Take it slow… make it ALL about her… and communicate (and not just pillow talk… Real talks without jumping each other).
Good luck sweety :-)