My dream team of FE at it's bare minimum would be Marth, Lyn/Hector, and Robin.
Marth started it all, no brainer. Lyn/Hector mostly because FE7 was the international bridging gap between a Japan-exclusive and internationality as a series. Lyn/Hector are equally interchangeable and could be either/or for whomever you wish to pick, since they're both unique in their own ways and have their own moveset appeal. Robin is the most accurate to FE's game mechanics, a mage user, a representative of the new Fire Emblem gimmick of the avatar protagonist, and from the game that revived FE as a series when it was at its lowest. Robin and FE13 are both very important, and I might argue that Lucina and Chrom are also viable if they give a damn to update their movesets to be less of a clone. FE15 is really up to Sakurai, since it's routine to shill the newest installment of the series like Pokemon at this point.
Personally, as a Golden Deer, I see a lot of potential in the likes of Claude but I know damn well that will never happen.