Not saying I personally want to see all of these archetypes, but here's how I'd imagine they would work.
okay in order
1 how do they differ both mechanically and thematically from other bards?
Perhaps they can actually use music to create certain effects over periods of time--less useful in battle, more useful in social situations. Play music in an area for a minute or ten minutes and thread a
suggestion spell in with the music, affecting everyone who can hear it. Obviously, this wouldn't be a good archetype for most dungeon crawls, but for more social games it could be quite effective.
3 how do you make it not teifling default or hell warlock is the real question.
WotC, in their infinite lack of wisdom, made tieflings Asmodeus spawn. A fiendish sorcerer could gain different powers or bonus spells if they're linked to devils, demons, yugoloths, or whatever else type of fiend they want to use. And all fiendlocks are fire-based, even though plenty of fiends, and plenty of layers of the lower planes, are more based on cold, acid, or poison.
4 a that would make the monk even more MAD as abyss also why are all monk subclasses pure fighting styles it is kinda dull but could work would need some offence abilities to not be dull playing them.
If you allow Strength to replace Dexterity or Wisdom for this archetype, this shouldn't make the class any MADder.
5 how would they not just overlap with the necromancer or the two undead warlocks?
Perhaps give them the ability the cast spells through the undead they make? And/or an ability like that of the spore druid that lets them create temporary undead without casting a spell.
6 okay the thematic could work but what does it bring to the druid not presently there?
It would be a more social-based druid; as it is, druids are
not face characters.
7 would be MAD as anything also other than stat change what does it bring?
People have been wanting a Strength-based pugilist for some time. Flavor it as a boxer and it would work quite well, especially if they could use sneak attacks while punching. (Hey, if Swashbucklers can get sneak attacks while dueling, then why not Boxers?)
8 that would need lots of spells being made first.
Since when has D&D shied away from making spells? Especially when there's so many spells from previous editions that haven't been converted yet.
9 how would that even play?
OK, this one I'm not seeing either (assuming you're talking about the feral monk built for animalistic races). But a monk who could sprout animalistic claws and fangs--a weremonk, if you will--could be pretty cool.
10 overlaps a lot with the new barbarian subclass also how do we deal with it being unusable by phb races?
I'm not sure which barbarian subclass you're talking about
11 we have too many magic fighter subclasses as there are at this point we need the arcane gish just so people stop cluttering up the fighter.
I think having "too many" of a type of subclass is just an opinion here.
I also, personally, don't think an arcane gish is going to happen. If the eldritch knight isn't enough, then I don't think that anyone will be able to agree on an archetype that does the same thing but "better," and I doubt they'd be adding yet another half-caster class.
12 okay but mechanically what does it add also we lack sufficient plant monster for that to work right now.
There's actually quite a large number of plant creatures. Plus, if you don't turn into specific creatures but get plant-like features, you don't need to worry about not having a million plant creatures. Most terrains have plants in them, so controlling existing plants would be easy--and the archetype could include a feature where you use your action to plant a seed (of any type) and it instantly grows into a plant you can control.
13 we already have it was in swords coast adventure guide.
That one is true. Oath of the Crown needs to be reprinted and maybe updated.
define setting specific that is not just a slight variation of a present subclass?
Well, the archetypes that got cut from Strixhaven are prime examples.
psionics needs a full class first before we really make subclasses as at present they will just end up with more quarter casters.
That will almost certainly not happen this edition.