These are my initial thoughts, but I'm eager to hear what everyone else thinks. If there's something people really want, I will see about implementing it, even if it involves a bit of reworking to get it functioning correctly.
Datapacks:
1. If the structures generate in that way, then I agree with Cyphur that it'll likely cause problems with world guard etc, as well as being easy to abuse. There's some tiered loot in the new reward bags I added for voting, which I could add to the random pool for dungeon etc chests if people want. Furthermore, I'm working on a couple of plugins at the moment which add more structures with cool loot and bosses, as well as methods of getting different gear based on what you want.
2. This could work, but depending on the exact implementation of this I'm not sure how functional it will be. I've seen some datapacks/plugins which add custom enchants in a very poor way. Again though, if people want it I can try and get it working. Similar to point 1, I'm working on a plugin at the moment which adds some of these features in a slightly different way which may work well.
3. I'm not against this, adding new things to the fishing loot table could be fun. I'd have to test to see how the custom mobs work though, make sure it doesn't cause any problems.
4. I'm all for new advancements. I've been trying to add a bunch for a while now, there's a set on one of my test servers which seems to work alright. Not too sure about the recipes though, looking at that list there's a few on there I think we might want to remove. I'll try and get my previous attempt at advancements more polished in the next few days and see about adding some of these too.
As a general note on datapacks: Spigot is really fussy about everything, and datapacks are no exception to this. The interface between datapacks and plugins is a really weird one, which may cause all sorts of problems that wouldn't be noticed straight away. I can't promise any data pack will work correctly (or at all) on VC with all its plugins, however if there's one which is highly requested it would probably only take me a day or so to convert it over to a plugin which works.
Plugins:
1. As with datapack 2, it could work, but the exact implementation in the plugin will affect how smooth it is. Again, I'm working on a plugin for VC which adds some of these features in a slightly different way.
2. I'm not too keen on this personally, mainly because I'm not sure how it would fit in with VC. At the moment, I'm trying to make a couple of different plugins which add some features from various mods, and it's really hard to keep the balance between new content and vanilla compatibility. I'm all for adding new content, and throughout lockdown I've made a big long list of stuff I'd like to try and add, some of which I'm working on now. While things like this *could* be fun, there is a very fine line to draw when adding these. With the projects I'm working on, my goal is to add the content in a way which best interfaces with the mostly vanilla style VC currently has, although if people want a less vanilla style I'm not against that, I just feel it needs to be done carefully