With the introduction of Vanguard Ike as an legendary hero, the floodgates are open for others to have the same treatment. For me, I would consider some being a legendary hero if they have a massive influence over the plots of their game or have a high authority in game. Some of the ideas for future candidates I had are,
Hero King Marth: by the end of Mystery, he had become the leader of the United Kingdom of Archanea, quite a feat.
Knight Lord Seliph: He had avenged his father death, stopped the Grannvale empire, and put an end of the Loptyr Cult.
Athos: He is one of the Eight Legends in Elibe, and has the stats to back it up
Dheginsea: King of Goldoa, and one of Ashera's Three Heroes.
Sephiran/Lehran: Bird Jesus that caused majority of events in Tellius.
Light Priestess Micaiah: Where she is empowed by Yune
The main trio of the Blazing Blade
Great Lord Chrom
Hero/Conqueror Alm
Master Lord Roy