I'm looking for a word to describe a person who leaves a legacy. It would be nice to call someone or a group of people that word. Similar to how being a maverick is someone who's different from the rest. I thought legateer would work, but this word doesn't exist.
The legal term here is...
testator - a person who dies leaving a will or testament in force
...but you'd rarely hear that in normal conversational contexts. Ordinary people don't have a word for "person who died leaving a will" - presumably because there's little need for it outside of legal contexts.
BUT - if we move slightly away from the specific context of "dead person's last will and testament", there will often be contexts where the appropriate word is...
someone or something that provides help or an advantage : one that confers a benefit
especially : a person who makes a gift or bequest
Also, you may want to look at the differences between legatee, devisee and beneficiary
Beneficiary– a person entitled to any part or all of an estate. Legatee– a person designated by a will to receive a transfer of personal property. Devisee– a person designated by a will to receive a transfer of real property.