I’m going to try to use Algonquian languages to illustrate. The language at the top of the list are living languages.
The middle ones stopped evolving as soon as they were written down. With scholarship and hard work, one theoretically speak them well enough to talk to the ghosts.
The ones at the bottom of the list are so poorly attested, that it will take a creative act to bring them back to life. This creative act is informed by science and scholarship, but in the end is a creative act of deciding which features and words to borrow from the languages on the top of the list. This is at once tragic and a bit
At the top of the continuum are threatened languages, i.e. they are still spoken by a small community. The living Eastern Algonquian languages are: Micmac, Maliseet/Passamaquoddy
* Cousin languages. There are other Algonquin languages outside of the eastern group, that are still alive.
These languages may provide clues or inspiration for reconstruction. These are good sources because they are likely to be either well documented to modern standards. On the otherhand, languages are a moving target, they evolve and the living languages of today are 200 and 400 years worth of changes different from what they used to be.
Documented, but dead, revivable.
Wampanoag – currently being revived, well documented.
* Proto Algonquian. This is the parent language to all Algonquian languages. Exctinct, but it is reasonably well inferred, even though it isn’t documented in the sense of having a grammar written by a linguist who spoke with the proto-language speakers.
* Cousin languages. Again, another source of material for reconstruction.
The documented but dead languages may in fact, maybe a better source since a reconstructed ghost-approved language would have features seen in the language 100 and 200 years ago.
Extinct, but at various degrees of documentation
Abenaki. Documented. More abenaki material.
Pequot. some people interested in revival
Wampano – Quiripi – Some published work by Rudes to revive it It looks like a “The Complete Language Guide for Learning, Speaking, and Writing the PEA-A WAMPANO-QUIRIPI R-DIALECT” has been published in 2007. Available on CD.
Mohican. Extinct, but has a yahoo mailing list. Also the Mohegan language reclamation project. (Sorry if I’m confusing languages here, there seems to be mutliple names for multiple languages)
Munsee- Down the the last 2 speakers. Looks pretty well documented, has online dictionary. This language was also spoken by people in Oklahoma, so it looks like there are two revival efforts going on.
Nanticoke. Extinct. Only ~146 word list.
Piscataway. Extinct. No records?
Carolina Algonquian. Extinct. No records?
Shinnecock, aka Algonquian Y dialect. No records?
Powatan – Extinct. ~600 attested words.