Looking for some Ideas - What should I add in my time travel mod?

you need muskets

make it a ranged weapon with a really long cool down

I remember there being a mod back in 1.7.10 where there were some weapons like a musket, a blunderbuss an even a blowdart amongst some other things. I think this was it: Balkon's WeaponMod.

So, like in this mod, you could add a Musket item with a long "pulling" or rather charging time, and of course ammo and it would shoot out a fast projectile. It was a really good mid-game ranged weapon back in the day, with high damage, but a long cooldown. If you'd like, you can check out that mod for inspiration.

If you can, you could maybe give the Primitive villagers a sort of bartering system like piglins have, indicating that they are trading things and do not yet know the concept of currency. I am unfortunately not good with coding, so I don't even know if something like this could be possible, but it would be neat.

Maybe in the past eras, there could be much less coal generating, indicating that it formed over-time. I like the musket idea, it would fit into a closer future better.

When I will have some free-time, I will test your mod and give feedback based on that.

Do you have a discord for this?

We should colab together, I could help fill out your addon I've been working on a similar style of addon concept. focusing on cryogenics and deactivation and reanimation of mobs modern world and historical 

Now, I have done some testing and here is my review.

-Very Far Past-
> The volcano structures are a bit off.
> There should be at least one resource that makes it worth coming here like let's say Sooth, Ash or Sulfur (I am leaning towards Sulfur, but it's your mod)
> It would be good to give some kind of use to Lava Rock, but I like that volcanos are made from this, instead of just some generic vanilla stone

-Far Past-
> Trilobites are very common and drop much scute. I think the armor is way too OP for such common drop
> Trilobite spawns are so common(HUNDREDS within a 3 Chunk radius), that it lags very easily
> Ancient Sand does not have a use, the very least you could perhaps smelt it into Glass

-Past-
> Making Leaves compostable. As most vanilla leaves has a 30% compostability, I would say that's good for this too
> Maybe a Hazel fruit/nut whatever you call it, would be nice. Maybe dropped with a chance from leaves?
> The Silver birch biome is good... but it's basicallya normal Birch forest(minus the flowers), so maybe a Birch-like new tree would be cool.
> While Wild carrot is a good idea, it is very rare and has no use. It could maybe drop regular carrot instead of itself when broken or some kind of Wild Carrot or the plant itself could have some use
> Auroch furs have no use either and they spawn in too big herds
> In the Advancement "The Last Bull", you don't have to put your text into ""
> The Primitive Villagers' structure causes them to spawn inside a 1 block of terrain, meaning that whil they don't get damage from this, they are stuck inside
> Wild Boars doesn't have drops, just xp
> You could maybe add Dodo or some other bird

Auroch AI bugs and strange things:
> After switching from Survival to Creative, some Auroch doesn't lose aggro for some reason and keep following you.
> Normally, aurochs follow a randomly selected mob around. If that mob dies, they select a new one. As far as I can see, the new mob can never be an another auroch. They only wander around when there is no mob in their follow range to follow. This doesn't seem to affect players

-Near Future-
> You could add some ruins for immersion

-Future-
> You could add more shrubbery and vines to the Overgrowth Forest biome
> Ashen wasteland lacks ashes. It's not neccessary, but it still lacks features despite being a wasteland

Conclusion:
There are some imbalances and there should be at least one extra resource per Time. All biomes lack features, a bit of extra "life" could spice them up more.
Many new additions lack use and common tags and loot chests lack loot tables. Despite the hardness to make such an item as a Time Jumper, there is not much incentive to do so.
I like these ideas and it has potential, but in it's current state, this mod needs more polish.