1) irrigation viaducts. a series of tiles which would extend from a river through farmland adding value to a farm?
I like this idea. We discussed using this in the Roman Influence expansion, but we had some difficulty figuring out how to make it work. We went back and forth between making these tile features or using meeples (like Catan roads or something like them) to build viaducts. In fact, if you look at the expansions in development thread, you'll see that the tile thumbnail for the expansion is a viaduct tile, though this tile is not actually included in the set as it stands currently.
2) use pig meeps as wild boar in forest tiles which could add value for the Hunter (Log Cabin) expansion.
This is an interesting idea. I wonder if it could be done as an adaptation of Shepherds to the forest. You could use the pig meeples from Agricola (the same set that included the sheep) and a spare meeple as a hunter and use the same, or similar mechanics from the shepherds in the forests instead of on farms. I like it, let's work on it.
3) A pigeon rookery which when placed in a city offsets siege (were often used as a food source which flew out to feed of the land and returned.
I think the Cathedral effectually does this already, though there are only two of them. However, there are four Siege and four Cather tiles, though I doubt they are played together very often, as well and a few here and there in fan expansions. I guess it wouldn't hurt to include something to couteract the Siege, but it wouldn't be entirely necessary.
4) Some sort of tile that could introduce locusts to fields.
This is also an idea that we explored in the Natural Disasters expansion, but didn't make the final cut. That tile set is only six tiles, but might have been 12 and this is one that would likely have been included.
