It's hard to give you a list of these RPG games, in estimate their is 110 listed RPGs on the PS2 alone
For Suikoden I haven't played it but I believe they are seperate stories just like the Tales Series and Final fantasy.
Their is DragonQuest VIII for PS2 it one of the best in the series. Though voice overs rough and static. In DS you can get III IV, Joker, and Joker 2, of the series which are real great as well.
You could try the Drakengard Series.especially if you looking for a good Hack and Slash game and I love the story of it. Though I war ya it is a very dark game.
If you played both Series of .Hack// (the Original IMOQ and G.U.) then my bad, but haven't played the one of the other I seriously say you play it.
Their is also the Wild Arm Series, which is almost like Final Fantasy, except it is more Western Based (cowboys and indians like) then the semi Medieval / Steampunk games like FF. Really it is a fun series, has lots of good things and can give. 3 is like a normal Turnbase but 4 and 5 has Hexagon feild which makes the games unique to fight in.
If you want a more sutle game, you could try the Champions of Norath and CoN: Return to Arms. Its more slashing game but it is also a multiplayer RPG which is rarely seen. Getting a friend over and both of you can create a character and fight.
Xenosaga series is pretty much one of the best jrpgs for PS2, if you haven't gotten it you missing out a bit. Its real fun, very much like Final Fantasy as well, but it is more Sci-Fi then it.
If you like Star Ocean Last Hope, then you would like Star Ocean: Til End of Time which I heard is one of the best of the series (I sure like it). It has a nice story and some good plays. It's real fun.
Then if you want more tactical, their is Disgaea and Disgaea 2, which is a fun game, where you make characters and fight. Also one based of it with the removal of grid system Phantom Brave which is also a great game.
Their is more I could give you Eternal Sonata, Radiata Stories, Records of Argarest War, Breath of Fire, Neir, etc. but running out of space to talk about them.