Realistically speeking, IF LH wants to cover the connecting market from GLA itself, they'll have to do it from one of their hubs as close as possible to LHR to avoid having pax make a huge detour, so I don't think well see a GLA-MUC or GLA-ZRH flight anytime soon.
Only possibilities are FRA (once the new RWY opens) or BRU, possibly DUS.
My money is on FRA, unless SN wants to give GLA a try once more. They did it a few years ago, and failed, but this time, they could use BD regional as operating carrier, just like they do on the trice daily service from BRU to BRS and NCL (and with some success, it seems)