Chachalote made the very good point that Lego and Meccano are construction toys from the initial concept, but Playmobil was designed to be focussed on the figures from the very outset.
Adult collectors round the world, with the luxury of large disposable income, have created wonderful massive constructions with Playmobil - the River Steamboat, the train station, and any number of customised castles all come to mind - but are most children interested in using Playmobil in this fashion?
I find the construction part of a set tedious. It's worse with system-x than with steck, because even before clipping the parts together you've got to clip the clips into the parts for 30 minutes or more, and with many sets I've grabbed out the klickies and dressed them up, and left the building construction till later, if at all.
When I had Playmobil as a child, and I played actively with Playmobil for 5-6 years, I never had any buildings or vehicles. Just figures, horses, and lots of accessories. My castles were stacks of books, the space under the bed a pirate's treasure cave, cardboard boxes were western buildings. There's a couple of young guys over on Playmoboard, Pedroskivich, and his friend Nate, who aged 13 are still doing just this, and having great medieval battles.
Even now when I have the luxury of money to buy the big expensive building sets and vehicles, I still get more pleasure from setting up the figures than creating a custom building. I think most parents don't buy endless add-on sets for their children to construct ever larger buildings. One or two buildings as a centrepiece and a few figures to play out a story, and that's your lot, son.
The desire to construct custom buildings, and thus the need for a modular building system into which everything must fit, is not something I think children want from Playmobil. They could get that from Lego (and don't forget, children are quite happy to mix and match toy brands far more then we blinkered adults).
I would be happy to see an truly modular building system as part of the current range, preferably in steck, with parts available as add-ons to expand. I'd like to see castle parts made in the same pale limestone colour as the lighthouse, so we coud have some Caribbean forts, and new pieces like a pepperpot sentry box, so we can make those forts more realistically represent to the originals; not just in the Caribbean as we've seen, but around the Mediterranean and the Holy Land limestone would be better than the light and dark greys of the current castles. Let's also red brick so we can build Chinese castles.
While all this would be wonderful, and the adult market might snap these ideas up, I don't think the vast majority of the Playmobil using public - the under 13s, would be bothered with such things. The ranges are designed with one large central set (if you're the lucky child of rich parents), and various smaller sets at a range of prices to round out the theme.
So to return to the Lighthouse set, this is the initial centre piece of a currently small sub-theme to the pirate world. If it sells, then maybe there'll be a bigger fort for the Conquistadores to defend - or maybe we'll have the Conquistadores set out into the South American jungles and discover the Incan and Aztec temples..