How the UEFA Nations League could line up

The UEFA Nations League kicks off in September 2018 with UEFA’s 55 member associations split into four leagues: but who will be in which?

Who goes into which league?

The 55 participating teams are split for the inaugural edition into four leagues, A–B–C–D, according to their position in the UEFA National Team Cofficient Rankings following the end of European Qualifiers for the 2018 FIFA World Cup (as per 15 November 2017).
League A will include the 12 top-ranked teams, the next 12 teams will be in League B, the next 15 in League C and the remaining 16 in League D.
Leagues A and B will consist of four groups of three teams
League C will comprise one group of three teams and three groups of four sides
League D will be formed by four groups of four teams
All draw procedures to be confirmed
After the initial edition begins there will be promotion and relegation between the leagues, and the ongoing UEFA Nations League rankings will determine seedings for all future European Qualifiers.
FULL DETAILS: ALL YOU NEED TO KNOW

How would the leagues look if they were decided today?

The latest Interim National Team Coefficient Rankings were published after the most recent European Qualifiers on 26 March 2017 (not to be confused with the separate FIFA rankings). If the leagues were decided by these rankings today, here is how they would look …

LEAGUE A (teams to be split into four groups of three)
Germany, France, Portugal, Belgium, Spain, England, Switzerland, Italy, Croatia, Poland, Iceland, Bosnia and Herzegovina

LEAGUE B (teams to be split into four groups of three)
Republic of Ireland, Russia, Austria, Sweden, Slovakia, Wales, Ukraine, Netherlands, Northern Ireland, Czech Republic, Turkey, Hungary

LEAGUE C (teams to be split into one group of three, and three groups of four)
Greece, Romania, Slovenia, Israel, Serbia, Denmark, Bulgaria, Albania, Scotland, Montenegro, Norway, Lithuania, Azerbaijan, Cyprus, Armenia

LEAGUE D (teams to be split into four groups of four)
Estonia, Finland, Belarus, Georgia, Faroe Islands, Latvia, FYR Macedonia, Kazakhstan, Moldova, Luxembourg, Liechtenstein, Kosovo, Malta, Andorra, San Marino, Gibraltar

These leagues are provisional, and the line-ups will only be confirmed after European Qualifying ends in November, with plenty of opportunity for teams to move up and down the categories.

However, as things stand …

Belgium, Switzerland, Croatia, Poland, Iceland and Bosnia and Herzegovina would all have a chance of a first major national-team title competing in League A, with the winners decided in a final four tournament between the group victors in June 2019.
Former European champions the Netherlands and UEFA EURO 2016 semi-finalists Wales are among those currently set for League B.
EURO qualifiers Romania and Albania could be in a competitive-looking League C.
One team from League D would be assured a place at UEFA EURO 2020 via play-offs that March; of those, only Latvia have previously made a major final tournament, UEFA EURO 2004.

Note that Russia and Ukraine can be in the same League, but would not be drawn into the same group. The same applies to Armenia and Azerbaijan, if the current UEFA Executive Committee decisions still apply when the draw is made.