LEGO never like to let a good, or successful, idea go to waste. Fans are often disappointed when a theme is only available for a year and then disappears, but at least there is the comforting knowledge that it will probably be back. Renamed, changed a bit, but back none the less.
An obvious example of this is the Castle theme. This theme has never really gone away, but has been re-named over the years and had a variety of factions introduced. Castle became Knight Kingdom, reverted to Castle before its current incarnation as Kingdoms. These iterations were not equally popular, but the theme has always been around.
Much more common is for similar themes to pop up from time to time, with a few years in between. A good example of this are the underwater adventure themes that LEGO have released.
In 2010 LEGO Atlantis was extremely popular as the big, heavily promoted key action-packed theme of the year, and it continued into 2011. The premise was that a group of deep-sea divers were collecting to keys to unlock the Lost City of Atlantis, but had to overcome enemies in the form of the Atlantis Warriors. The heroes use a variety of futuristic submarines and technology to explore the underwater world, and the villains are accompanied by creepy underwater creatures.
The LEGO marketing team were keen to emphasise that Atlantis was all-new, but it had a lot of influence going back to the Aquazone line that had been introduced in 1995. Aquazone was based around the Aquanauts, who were mining crystals underwater and used yellow high-tech underwater vehicles, having to combat the Aquasharks (who also wanted the crystals) in their red and black shark-inspired submarines. The theme introduced new elements including the sting-ray and octopus, both of which have been used in other themes since. In 1998, Aquazone had a refresh with the introduction of Hydronauts as the protagonists and Stingrays as the antagonists, much in the same way the Blue Coats were replaced by Red Coats in the Pirates theme.
But when the Aquazone theme was discontinued in 1998, it was not the last underwater theme until Atlantis. LEGO Aqua Raiders was released as one-off theme in 2007, and shows the transition from the designs of Aquazone to those of Atlantis. The subs and tech are yellow, like that of the Aquanauts from Aquazone, but the huge brick built sea creatures a precursor to those that would be seen in Atlantis. The use of magnets is also a throwback to Aquazone, and this theme does feel like it would fit right at home alongside those sets (although it is perhaps a little less other-wordly than Aquazone).
What these underwater ranges show is that LEGO will revisit a common idea time after time. Sometimes there will be quite radical departures from previous design aesthetics, sometimes a slight shift as there was from Aquazone to Aqua Raiders. What this does show is that in all likelihood LEGO fans can expect a new underwater theme in a few years’ time.