Good to see Transavia back after such a long gap, hopefully it proves popular. Also good to see some larger, more reputable carriers in the mix at STN with Sun Express and Royal Jordanian joining in the past year, as opposed to much smaller outfits like Air Albania and Air Moldova seen previously.
ORY is still one I could see working for Transavia at the right timings/frequency. The Eurostar is undoubtedly a major factor, but it isn't a show-stopper for a potential Paris route to work. Given LHR, LGW and LTN can sustain regular Paris routes, I don't see why STN couldn't. In fact, given LTN and LGW's catchment have slightly better rail connectivity to St Pancras compared to the East-end and Essex, a STN route would be quite an attractive offer to those areas. EZY did operate STN-CDG in 2019 up to 6x weekly but was a bit of a half-hearted effort given their general lack of interest in STN, but that could still be theorectically possible with EJU operated frames.
KLM Amsterdam isn't totally out of the question but I think unlikely for the forseeable, given slot constraints. They'd also have to be confident it wouldn't dilute LCY and NWI loads, plus European hub connectivity from STN generally won't be as attractive to the higher yielding traffic that uses LCY and LHR. They'd also likely need an ability to night-stop to meet hub waves in AMS, which STN would struggle to accommodate at the moment. Could certaintly see them a better fit for STN over LGW however if they did ever venture to a third London airport, given EasyJet's fairly limited frequency compared to LGW for the point-to-point traffic.
SAS operated CPH in 2019 after disposing one of their LHR slots, but never returned after Covid. They've already announced an array of new routes for S25 and their strategic focus has shifted since then. Last few slot reports have indicated they've requested LGW slots over STN.
Vueling Barcelona 2x daily I could see a future possibility, especially if the LHR slots they've been operating this summer transfer to BA.
Eurowings I'd love to see come back given Ryanair's retrenching in Germany and given the vast German network STN used to support under Germanwings/Air Berlin - but I can't see that happening with the fairly sizeable LHR operation they now have.
Turkish mainline and Qatar I think are only a matter of time but it will only be when they can't get additional slots or upguage airframes in LHR and LGW. Qatar especially have managed to secure extra LHR slots from TAROM and are now exceeding Emirates frequency at LHR, which given their OneWorld partnership is a major win for them.
Others I could see are Biman Bangladesh. I can see them getting priced out of LHR and relocating to STN which serves the Bangladeshi community in East London just as well for what's a heavily point-to-point route. MAG will almost certainly be targeting that. The current Cyprus Airways should also be a target, there's a long-standing history on that route and they currently don't serve London. One of the issues I see with that currently is any potential onward connectivity from Cyprus to the Middle-East being hampred by the political events there, so would likely need to attract more of a leisure based market for Cyprus - Interestingly they have LGW listed in their booking engine but no flights available.