There was speculation about a Phantasy Star 5 well before Phantasy Star 4 was released in Japan on the Mega Drive in December 1993 (We didn't get PSIV in the U.S. until February 1995, and yes I know the copyright date on the title screen is 1994, but that doesn't mean it arrived in U.S. stores that year.
It was 2 months into the new year before PSIV reached retail, and .... we payed $99.99 on top of it.
Also remember that originally, PS IV was going to be a Mega CD / Sega CD game with the subtitle "The Return of Alis".
So the best chance there was for a PS5 was on the 32-bit Saturn.
Here is something I found in Electronic Gaming Monthly issue 43 - February 1993 - Letters to the Editor:


At that time, it was well known Sega was developing more than one 32-bit system, but the name Saturn was only announced later in 1993. Phantasy Star 5 could've easily been more successful in Japan than the 16-bit Mega Drive / Genesis games, because Saturn itself was a big success in Japan, outselling the Nintendo64 and only outsold by the PS1, after Saturn had the early lead. With PSIV's December 1993 release in Japan, I figure 3 years would've been enough for the PS team, under Sonic Team, to develop a large 32-bit RPG, so by late 1996. However, even if Phantasy Star 5 had been made and released in Japan, an English version would've come too late for Saturn in the U.S. because SoA CEO Bernie Stolar famously said at E3 1997 "Saturn is not our future" while behind the scenes SoA and SoJ were each working on separate next-gen prototype consoles that eventually lead to the SoJ developed console being chosen and Dreamcast was announced the following year. As we know, that didn't lead to Phantasy Star 5, but PSO instead.