Lego has made tons of themes. Most were good but some were disasters. SO now I ask which theme would you want to replace. Personally I'd say
- Kingdoms with Vikings
- SpongeBob SquarePants with a simpsons or a smurfs theme theme
- architecture with a theme that actually has models outside the USA
- Atlantis with Greek theme
- Lego Racers with a theme with large cars
- Star Wars with LOTR
Which themes would you like to replace?