It's true that kids are easier tempted to buy from all kinds of themes, but I must say that thinking back about it I wish I did stick with fewer themes.
But still, there is a great amount of randomness possible within a single theme, and I would really like Playmobil that explore that and come up with less obvious choices within such a theme instead of always redoing the same few characters.
Any theme would in theory be capable of filling an entire playmobil catalogue after all. For modern its pretty obvious of course, and thats all compatible. But same is potentially possible for other themes. They shouldnt do that of course, but I just like to point out they have an equal range.
For example western.. within that you could have the themes: Train, ranch, Fort (north vs south), mines, town, Indians, Mexicans, Riverboat.
Or medieval: Castle (with interiour sets sold seperatly) village, forest, Stables, Armies, Merchants and trade, Crusaders, Arabs, Vikings, Highlanders, Religion, Farm.
Or pirates: Pirates, Asian pirates, Redcoats, Conquistadors, Palace, ghost pirates, trade, tropical island (with natives), Fort.
Maybe even space: Monorail, mine, city, evil aliens, very advanced intelligent aliens, bandits and space police, explorers, space station, space port.
Providing a wider range of figures within a theme just expands the play possibilities for a child. Give a kid some knights and soldiers of two factions and you know he will play a battle with them. Provide some peasants, and then the peasants could be protected by one army. Add a princess and knights could fight for her favor, or she could be kidnapped. Add a minstrel or jester and there might be some comic relief between the fighting. etc.
It simply allows children to really think further as they play and encourage more creativity. To us it might come natural to think of all the possibilities with a figure, but a child might require more encouragement.