The Nations League has been a wild and brilliant success, but will this be as good as it gets?

Promotion and relegation promise to change the tournament's tight competitive balance. Will it still be as intriguing second time around?

As this first phase of the inaugural Nations League season comes to a close, it is hard not to conclude that in a world of terrible ideas on how to change or improve football, Uefa have found one that works.

At the very least, the new competition has achieved its stated aim of replacing meaningless international friendlies with meaningful competitive matches.

Any impartial observer to Switzerland’s 5-2 comeback win against Belgium this weekend, the Netherlands’ late draw to qualify in Germany on Monday, or even England’s wins against Croatia and Spain can attest to that much.

The competition has also maintained the momentum in storylines first established at the summer's World Cup, ones that would have been easily lost in the familiar slog of a routine qualifying campaign.

England's rise and Germany's demise are two obvious talking points, though Spain's strange, inconsistent start to life under Luis Enrique poses questions too, while the Iceland epic appears to be reaching a painful conclusion.

Elsewhere, three World Cup semi-finalists - France, Croatia and Belgium - failed to live up to the high standards they set in the summer by either securing mid-table obscurity or suffering relegation. As a consequence, even Didier Deschamps' world champions face fresh, pertinent questions.

In the Netherlands and Italy, meanwhile, there is a renewed belief that they can compete with the very best at this level despite failing to qualify for Russia 2018 and the long post-mortems which followed.

All this intrigue could be found in 'League A', the top tier of nations. There was more still in Leagues B, C and D, where regular, competitive football against opponents of a similar standard has lit a fire under national sides that were previously plodding along and put their strengths and limitations into sharp relief.

It turns out pitting evenly-matched teams against each other in high-stakes matches works well. The question now is: can it last, or will promotion and relegation begin to skew the Nations League's fine competitive balance?

After all, this first set of 'leagues' and 'groups' were seeded according to coefficients calculated over the best part of four years, ensuring evenly-weighted match-ups across the tournament.

The second Nations League season will start with several national sides 'out of their league', however, on the basis of just four results.

To take the most obvious example, Germany will expect to quite easily win promotion from League B in 2020, barring a cruel draw or terminal decline.

Joachim Low - or his successor - will privately back their side to come out on top against the likes of Austria (last World Cup qualification: 1998), Wales (1958) or Finland (never), no matter how handy those second-tier nations prove to be.

In League A, meanwhile, Bosnia and Herzegovina will deservedly take a place among Europe's elite after topping a group containing Austria and Northern Ireland, though the very best are unlikely to be too intimidated by a side which ranked 37th in the world this time last year.

This may seem a mean-spirited point to make when it is an obvious consequence of the promotion and relegation element, one of the Nations League's big positives.

Yet the price of 'pro/rel' is that another of this format's great positives - close contests between national sides of a similar standard - becomes somewhat diluted.

Germany will be confident of bouncing back quickly (Getty)

There will still be surprises, of course. It may be that Germany's problems persist. It may be that Bosnia establish themselves as a first-rate national side, capable of holding their own among Europe's best. Maybe Gareth Southgate can do wrong. Maybe England will be relegated.

Equally, the Nations League's short format is conducive to upsets. One victory will still greatly enhance a smaller nation's chances of promotion. One poor result will still put an elite side in jeopardy. A long series of upsets could one day produce a hodgepodge tournament that sends an official in Nyon quickly searching for the reset button.

The likelihood, though, is that the competition will be a touch more predictable in its second season than it has proved in its first, when it began at a true competitive sweet spot, and it will perhaps be less of a spectacle as a result.