Hi Ras,
Since you've asked...here's a little list I prepared earlier ;-)
It appears that the most sought after items in the collector's opinion are also the most rare and sell for the most money on the second-hand market. But the ones that may appeal to a broader audience are ones that may match with current or imminent releases. I also don't think it always necessary (as in the case of the beer barrel) to release the whole set.
To match with the current zoo, I would suggest the Albino Elephant from 3809.
To match with the new pirate theme, perhaps the 3740 schooner.
To match with the new port theme, definitely the 3551 commercial fishing trawler.
To match the recent winter theme, the igloo could be incorporated by kids into play.
And to match with the new mansion, why not the victorian lamp-post.
Others that always seem to go for good money on eBay and could match into many current themes are the 3468 TVi Van for movie-making, 3448 Tavern which could be used as a house and the 3723 circus band could give concerts.
As for individual figures...the general concensus form this board seems to be for more pirates or Nuremberg guards. These figures may not appeal to kids but are of much less financial value making it easier for collectors to buy them in vast quantities.
One thing I would do though would be to make them in different, but still realistic, colours (except the Albino elephant and igloo obviously ;-)). This is in order that they may still appeal to the collectors who already have them.
I acknowledge that this list is not exhaustive, nor is it my personal wishlist, but it is simply meant to give an idea of how retired items or sets that are desired by collectors may also be able to appeal to a broader audience, all at the same time.
It would have been my idea that we use broad spectrum sets to hook Geobra, and then once the working relationship is established, go in and ask for the more specialised items like the windmill and water tower which may not sell in such vast quantities.
Claire