The dovecote is a really neat structure, being able to reliably generate eggs other than chicken eggs, and the homing pigeons have some cool applications in protecting keys or valuable items when you get jumped in the heartlands. However, the risk of bird lung and the tedium of keeping them clean mostly outweighs the novelty of they offer. I'd like to see them expanded to house magpies as well, maybe via either a second type of structure, or by seeding them with magpie eggs collected in the wild.
Rather than acting as homing pigeons, magpies could leave you little treasures they've found (or stolen) in the surrounding area. Maybe they could pull from something like a local loot pool, similar to how you can find despawned items when digging, or items dropped in the water when fishing. Whenever a dropped item despawns, they could have a chance of being picked up by the magpies. Depending on the quality of swill and birds, that chance could increase, the radius of the loot pool could increase, or you could apply filters to have them target more valuable items. I'd REALLY like to see them be able to rarely generate some naturally spawning shinies, like gold pebble or river/oyster pearls