How recently it was when people were wondering whether the Trump-Russia imbroglio might get to the Watergate stage! Now it looks very much as though it is rapidly shooting past that landmark. Watergate was purely domestic; this could produce charges of treason against a sitting President or at least his associates—really unprecedented.
As experts on these matters point out, when someone in an investigation environment is asking for immunity from prosecution, it means that they probably assume the fuzz are aiming to turn them in order to get testimony against someone higher up, and who was higher up than Flynn? Only Trump.
It appears that Flynn’s plea for immunity directed at the FBI and the Congressional committees is not piquing their interest much yet, but it’s still very early in their investigations. They don’t know how much they need him yet.
If he’s worried about being prosecuted for something, it could be the Russia thing, the Turkey thing, or some other thing in Flynn's history. What matters is what he can tell the FBI and/or the Congressional committees, and whether his testimony would be valuable enough to them to warrant grants of immunity. I.e., can they get the same information from other sources, so they don't care much about getting his testimony. I.e., they might very well be able to get all they need from more than one source including Flynn.
In other words, all that's needed to make this almost a carbon copy of Watergate is White House tapes of Trump recording himself like Nixon, and that's not too likely, given what it did for Dick. Whoever writes a book about this whole thing after it's over will have to write several volumes.