I'm not betting on any real True Rune appearance. The best we can get is Brandeau or the RoP's bearers before him. Hero4's situation may vary according to data-transfer (which we don't actually know if it will happen), so he may be either alive or dead, after the "during Suikoden 4" period.
As for Kooluk: we may get a glimpse. After all, they did
affect the Island Nations. Again, this depends on where the setting is. I won't count on it if the setting during S4 is Gaien, since this was the only real nation uninvolved in the war. There may be reference, though.
Kyril may not be fighting Kooluk, but still: there are pirates and possible rebellions (maybe the people of Gaien wanting to send some forces to Razril).
Based on interviews and game reviews we're going to see enough character history to make up for the lack in S4. Maybe we'll need more than one playthrough to understand it all correctly.