Free (or, no premium) international roaming and wi-fi calling on T-Mobile are now essential features for me.
One thing I've always wished for was for my phone to have a routable IP address and not be behind NAT. A&A offers this in the UK: http://aaisp.net/telecoms-mobile-data.html
I've enjoyed that for awhile now as well with T-Mobile. Except being in Japan more frequently as of late, the 2g speeds for free are great for essentials (messaging, maps, email) but damn I'd love a better package for LTE speeds that isn't like $50 for 200mb or whatever it is.
One thing I've always wished for was for my phone to have a routable IP address and not be behind NAT. A&A offers this in the UK: http://aaisp.net/telecoms-mobile-data.html