This isn't recent Akaneia history. Too much is different already. If there's one thing the character designs demonstrate, it's that society and technology have advanced, putting this game some time after FE1/3. Which means Marth should not be at the age he's displayed in the trailer, so either it isn't Marth or something happened to him - and either one seems necessary to explain why he seems different.
The thing is, if it's too closely dependent on FE1/3's story, there are concerns about how much sense it might make to people outside of Japan who haven't seen an FE3/12 release. Best-case scenario, this brings enough importance to an FE12 localization that it happens, perhaps later as a port. Worst-case scenario, it keeps FE13 from getting out of Japan.