I only now saw this thread so please forgive that I haven't read it all.
I've been living in the US for almost 2 years now. It's funny how sandwiches are very much a meal in themselves over here - often being 1000+ calorie monsters. I've always seen sandwiches as a light snack - or if part of a meal, to go with other things like fruit, yoghurt, vegetables, soup etc.
One of my personal favorites growing up was very simple (and probably sounds awful) - but strong, thinly sliced crumbly cheese with honey. The mix of sweet and sharp being soaked up by the bread worked well. Also I was a big fan of single slice grilled toast, with thin cheese and a bit of crumbled bacon stock powder - grilled ‘till the powder just starts to burn. And maybe some relish underneath the cheese too

.
In the US I find you often get an overload of competing flavours where quantity is the order of the day. Very thickly sliced meats, covered in thick, extremely sugary sauces. Enormous quantities of generally very mild cheese. The dressings are always applied *way* too generously for my liking too. I find it's often satisfying like cheap chocolate; it throws so much fat, sugar or salt at your tastebuds that you can't help but get a rush, - but with the quantities it ends up feeling gluttonous and often I feel a bit ill afterwards. The most recent sandwich I had ended up leaving my mouth feeling almost tingly due to how much salt it had.