The biggest reference of all, I think, will be revealed. That one, I believe, being the revelation of Giovanni's son, who I assume currently is Silver, the main rival in GSC.
IIRC, Silver is an orphan, and it's very improbable that he is Giovanni's son.
I do hope they have Jasmine either leave on a journey... Or talk about her experience in Sinnoh meeting another trainer just like you.
Being that G/S was 3 years after RBY, I'm hoping it's the same for HS/SS being three years after FRLG.. D/P/Pt being in between both games (Thus Jasmine would have come back from her Sinnoh journey), to make up for the lack of the Bird trio (Hoenn's story would be fourth).. And possibly making a reference to Team Galactic and how stupid they were... or something..
If that happens, Jasmine would disappear from the gym after you have beat her, It's technically impossible for her to make references about D/P, since she wouldn't be back from Sinnoh.
I highly doubt that they'll make a reference to Team Galactic, but it's possible they make one post-E4
The general theory that most people accept is...
Kanto and Hoenn take place around the same time
Johto and DPt take place around the same time
That seems to be how they're doing it.
Steven making a cameo in Platinum, hinted to be the guy who owned the Villa, makes me wonder if he'll appear in HG/SS.
I have my own theory(I'll be using the remakes as reference):
FR/LG goes first, after the game is finished Prof. Oak goes to Jhoto to visit Mr.Pokemon (I think that was the name of the guy who gives you the egg yo give it to Elm)
HG/SS happens 3 years after FR/LG or the Giovanni incident, Jasmine takes a trip to Sinnoh after you beat her, arriving a little before the Lake of Rage incident, the Hearthome hiker returns from his visit to Prof.Elm (This might be irrelevant), Oak goes to Sinnoh after the game is over.
D/P starts a couple of days/weeks/months after the Lake of Rage incident, Oak came to Sinnoh to check on the Pal Park and stays in the country
The only problem I see in this theory is that R/S could only go before R/B/G or some years after D/P (probably before D/P and between D/P and HG/SS, the Battle Frontier;which supposedly is in Kanto; suggests me that the Hoenn one was the first one, then they built it in Sinnoh, but this can cause the games to go in a double timeline, with the third installments forming the second one)
Steven's appearance in Jhoto would be pretty awesome, he is one of the best NPC's in the whole series