The other one that has totally disappeared from the radar as far as starting STN services is concerned is FlyPop. Not sure if they are still planning on officially launching, as it has been a long time since we heard any updates, but they had been flagged to begin a set of routes to India from STN, but since then, it has all gone quiet.
With some of the others mentioned... I'd love to see TK back (last served STN in 2009), and I think that KLM could well be a good fit for STN in order to boost AMS frequencies, something that EasyJet won't do now that STN is no longer a base for them. I bet you that if EZY dropped Amsterdam, KLM or Transavia would announce it pretty quickly. Cyprus Airways- potentially, although they will probably try LGW first before looking to STN. I did question in another forum whether Tarom would consider retaining a presence in the UK via STN, now that they will be exiting LHR at the end of the month, but sadly, I think this is unlikely, with FR comfortably serving OTP up to 5 times per day, well above Tarom's existing 1 daily flight to Bucharest from Heathrow, so I reckon they'll be withdrawing from the UK completely once LHR ends.
I'm also wondering whether Eurowings would consider returning to STN, now with FR reducing their presence in Germany. Germany has always been well served from STN, with the likes of Germanwings and Air Berlin serving the airport for many years, both of which are sadly no longer around.
On a positive note, it's wonderful to see Transavia back at STN after so many years away (last served circa 2006/7). Hopefully they will be successful at STN this time, and they will of course benefit by the fact that they will face no direct competition from FR to Rotterdam.
On a different note, any word at all on the terminal transformation updates?