I think Athens and maybe Shannon are the only realistic ones there.
The issue with Frankfurt is a mixture of high operating costs and slot constraints, with an element of restrictions in operating hours. There is no real incentive for the airport to do any special deals for the locos, it is pretty full without them and could undermine existing business.
Moscow would require extra slots to London for a Russian carrier. And tourism potential is constrained by onerous visa requirements. The EZY service uses the BMI rights BA were forced to give up. I would think LCY will see Moscow service long before LTN/STN would (when the C Series arrives)
Wizz won't touch Vienna, too close to Bratislava and not cheap. The route has come and gone from both LTN and STN in the past, it's not impossible but it wouldn't be the most obvious choice to start again
Scandinavia, I'm afraid LTN is at the back of a queue fronted by LGW and STN.
Waterford has been discussed to death