There was simply no symbol that would be even close to universal even for Scandinavia, let alone entire body of Germanic people (at least before they all converted to catholicism), because there was no point in history when they thought about all of themselves as one people. Same with Slavic, Romance etc. Language groups are not unified people. All tries to invent such symbols are only tied to XIX century+ pan-something movement and those are way too fashy to be comfortable with (and have no chance in hell as history proven already).