It has been suggested in multiple different places so I created this thread to stockpile the ideas. From what I understand, the main ideas have been to somehow link multiple legions together (whether by alliances, wars, or tiered multi-legions) and to increase the legion max size (to somewhere between 50 and 100 inclusive).

I'll also add a suggestion of my own: that even if the max legion size increases, the "legion bonus" scales the same, just with a 100% cap.
For example: Legion 1 has 45 members, so gets 90% planet stat bonus. Legion 2 has 50 members and that means 100%. When Legion 2 gains more members, the bonus stays at 100%, with the added security that even if they loose a few members while still over 50 they keep the max bonus.