When I look at pictures of Playmobil dioramas, I find the best ones include non-Playmobil scenery. Although PM rocks, grass, roads, trees, etc. do exist, sometimes a diorama can look a bit unfinished or incomplete if the creator uses only PM items. Just my opinion, but even Geobra use non-PM items in their promotional pictures and box art (this was especially true in the early days).
Perhaps the exception to this is when a creator produces indoor scenes (remember Toby's zoo visits and several other masterpieces by Raven?) which can look great with all-PM parts.
I agree with this - the early ones used lots of model-railway surfaces and plants. I feel like Playmobil trees are probably cripplingly expensive for anything on a big scale, but then I've not really bought model railway stuff either, so maybe it's the same sort of pricing.
When I was a kid I had a big table with a painted road and fields on it, but of course that does kind of limit what you can do unless you're prepared to regularly re-paint it