What you need for that is a herd of spiders, not plants! You could become a spider-herder!

Farming yes please

Farming as a skill line is a great idea! It would give homestead an extreme added purpose. Find seeds, fertilizer, water, and after tending to the plant for let's say 30 days it will start producing mats. Once the plant is fully grown you won't have to tend to it anymore and it will make mats every 24 hours. I would love to have a alchemy​garden as well as clothing plants and trees. Guess mining would be left out. Unless we could dig for resources? Idk. Highly doubt anything like this will happen. Our best bet would to hope for alchemy gardens.

I guess, all people here know how in TES V DLC Hearthfire gardening feature was implemented. In ESO it can be done in much similar way, with certain improvements. I don't know if it'll be an appropriate way to do such a thing with Ancestor Silk and other "non-consumable" crafting materials, but at least cultivation of the alchemical flora shouldn't be a big problem for economy. With properly calculated respawn time, of course.
I see it this way. First, you'll need fertile soil, which can be crafted or bought from vendors. Second, seeds, that can be with some relatively small probability obtained from plants (much like Alchemical Resin). Third, plant the seeds into fertile soil, that can be placed anywhere you want in your home. Finally, wait until plant will grow and harvest it.
After this cycle is complete, player need to find seeds again. It also would be reasonable if harvesting a home plant will have a much better chances to obtain seeds.

Regarding harvesting blacksmithing, clothier and woodworking materials from players estates. I can't see how leveling nature of such kind of materials can be taken into account and how it can be done in a right way.

Farming would be very nice. a size limit could effect what houses qualified for the item, i cant see that working in a 'small' house.

other idea- instead of crafting nodes, a delivery box. place box, wait X hours (maybe a day), and receive a delivery. it could function similar to a crafting hireling. there are already a number of ways to do this with passives, why not have furniture for it? get one crate for each craft you want mats for.

the problem here, is that even with a per-house placement limit, you can have a LOT of houses. so you have to consider the maximum scale that players will hit as soon as possible. it would need to be either an expensive vendor item (maybe acheivement for master harvester) or a crown store item. make it a Large item and you limit what houses it can fit inside somewhat.

There were several houses with water features that I would have chosen if I could actually FISH in them. Instead I went with the one in Rivenspire that's surrounded by water. My boat is kept in drydock in my backyard.

I think the farming element should be limited to provisioning and maybe, maybe, just maybe alchemy ingredients.

It's hard to search for specific ingredients, especially when you need them for provisioning writs. I need barley? Okay, I run every crate, every barrel and every sack, and I find stacks of everything but barley.
It would be nice if you could farm ingredients for recipes you regularly craft.

I wouldn't allow ancestor's silk (and ores, and logs) to be farmed though. As you say, you and everyone else will buy this in order to sell it to others. Now, I wouldn't call this Pay2Win, but it wouldn't be fair to have players spend money to earn a lot of extra gold ingame, for hardly any extra effort.
Even if everyone is doing it and silk prices suddenly drop, you can still extract it for some gold tempers and sell those.

This is why provisioning ingredients would be agreeable, and more realistic.