BRU, FRA and MUC
I think BRU, FRA and MUC only work as destinations if they're carrying connecting passengers (MUC maybe less so), including passengers connecting to long-haul. That tends to rule out Ryanair and Easyjet as operators, as I don't think they'd have the point-to-point volume to justify putting on a 737/319. I guess if that point to point traffic was there, they'd be operating them now - as Easyjet operate AMS alongside KLM.
It may be that the volumes on those three routes fall awkwardly outside the aircraft that LH Group has available to it, despite being potentially commercially sustainable with the right aircraft. Maybe bmi's 37/49 seat ERJs were too small (pushing up fares). KLM operates AMS with a 98 seat aircraft, Aer Lingus (admittedly competing head to head with Ryanair) uses 72 seat prop-aircraft on DUB. The 80-ish seat RJ 85's that SN were scheduling to operate BRS-BRU from May 2019 were withdrawn in 2016 because (as a 4 engine aircraft) they're really not economical to operate. SN operated BRS for many years (on a 3 x daily basis) using the RJ 85 - the move to bmi coincided with withdrawing the RJ's. LH did briefly operate BRS-FRA directly (using a wetleased RJ-100), but then pulled out.
I've emailed LH to ask them to look at options to get BRS back on their route map, but SN's apparent dithering over resuming BRS-BRU (which should be a reliable route, but maybe Brexit affected) suggests they don't have the right size aircraft to operate it, and LH may face the same problem.