Thameslink's been on the tube map before. It was present during most of the 1990s, initially in orange and later in white. Here it is in 1997.
Only the central section from Kentish Town to London Bridge and Elephant & Castle was shown. Also present at the time were the Northern City line out of Moorgate and the North London line from Richmond to North Woolwich. Thameslink vanished from the map in 2000, the year TfL was formed, with the North London line the sole surviving non-TfL service.
There are sound reasons for putting Thameslink back. It's the only non-tube line to cross central London, it operates a very frequent service, its fares are identical to those charged by TfL and a lot of people don't realise it exists. It might not even be too intrusive.
n.b. This is not an official map, this is me playing around by drawing lines. I'm using lime green solely because it stands out.
Thameslink now has two northern branches, one through Kentish Town and one through Finsbury Park both of which ought to appear. Fortunately most of the central section either shadows existing lines or doesn't really get in the way. The only additional station is at City Thameslink, which'd fit nicely into available white space.
But the tube map didn't have accessibility blobs in the 1990s and these could make interchanges very messy. King's Cross St Pancras is already served by six lines so might struggle with a seventh. Meanwhile Farringdon is due to get much more complex when Crossrail opens and Thameslink would only make that worse. Are we making the tube map more useful or just making it harder to follow?
And where do you stop?
Beyond Kentish Town the next station is West Hampstead, which on the map is way over to the left hence horribly impractical. Thameslink doesn't interchange with TfL services anywhere else in north London so it'd only get in the way. But it's south London which would see the greatest potential tangle, with at least four branches linking up to existing stations at Greenwich, Woolwich, Abbey Wood, Denmark Hill, Peckham Rye, Norwood Junction, East Croydon, Mitcham Junction and Wimbledon. Best not go there, I suspect.
You can see why TfL is "looking into" how Thameslink could be included on the tube map rather than committing one way or the other. Whatever they eventually decide, someone isn't going to like it.
As for whether or not the tube map is fit for purpose, here's a comments box for that: comments