International roaming used to be catastrophically expensive 100% of the time, but it's slightly less so now. T-Mobile helped pave the way for that when it started offering free 2G roaming in most countries. Leaked documents indicate that Tmo is planning a new add-on for its One plans that gives you unlimited international roaming, but it's spendy.
According to the leaked documents, T-Mobile will announce the new Global Plus plan option tomorrow (November 9th). It's only for those on One plans, and it includes all the features of the existing One Plus International add-on. The selling point for this plan is you also get unlimited high-speed data (so, usually LTE) along with 5GB of high-speed tethering and free calling. However, that'll cost you $50 per month. With the $70 base price, that works out to $120 per month on a single line.
Currently, customers on One plans can add a single-day international data pass for $5 that includes 512MB of LTE. Presumably, that will still be an option. Meanwhile, AT&T and Verizon both have $10 per day options. The Global Plus plan is not something you'd add and remove on a regular basis—T-Mobile wants you to keep it and pay $50 for international data every month. It really only makes sense if you're traveling out of the country a substantial amount of the time. Otherwise, that's $600 per year wasted.