In Pokemon Diamond, there were many things that hinted towards a G/S remake (which was made) some of these were
1. The Traveling Man who gives you the egg says that Pokemon eggs hatch when it is with energetic Pokemon, and mentions Professor Elm. A reference to Professor Elm of Johto, and how Pokemon eggs were introduced there.
2. The Pal Park Pokeballs are gold and silver with a crystal in the middle.
3. Next to the resort hotel, if you go into some guy's house, he mentions the Tin Tower, home of Ho-Oh. He also mentions Whirl Islands, home of Lugia. Finally, he mentions the dancing Clefairy, which you can see if you go to Mt. Moon on Friday.
4. Drifloon comes only on Friday, just as the dancing Clefairy. Lapras also only comes on Friday, too.
5. Kurt's unique Pokeballs are obtainable in the game, but only by hacking.
6. By hacking, you can get your Pokemon to be from Johto, meaning the game is trade-able with a Johto game...
7. Jasmine visits you, gives you Waterfall, and mentions that you should visit Johto sometime.
8. The SecretPotion says in its description that it's from Cianwood City.
9. At the beginning of the game, it mentions the red Gyarados from G/S/C.
10. Firered and Leafgreen were remakes of games needing them, so that leaves 2 more games to need them!
11. Crasher Wake and Chuck hold their arms in the same way.
12. After Bebe gives you an Eevee, she mentions Bill having a bunch of them.
13. There is a GS ball in the middle of every Pokemon Center.
14. Sweet Honey is similar to headbutting a tree in G/S/C.
15. Both games have a lighthouse that you need to do something in to make a gym leader appear.
16. The time thing all started in G/S/C.
17. The night guards are here again, who only fight you at night.
18. In the place where you get Shaymin, there is a Dragon Tamer who says,"It's been years since I've left the Dragon's Den" after you battle him.
I'm NOT saying this is correct but from what i've seen, i've picked up a few things which MAY hint towards Pokemon R/S remakes in Pokemon Heart Gold & Soul Silver;
1. Steven Stone appears in the game (Getting Hoenn starters)
2. Steven Stone also appears in an event (Latios & Latias)
3. Bill mentions Lanette and the Hoenn region
4. The Hoenn region is also mentiond by various people throughout the game
5. Kyogre & Groudon & Rayquaza are obtainable (Blue, Red & Jade Orb)
6. Hoenn Sound (Pokemon Radio Show)
these hints are similar to the ones in Pokemon Diamond & Pearl which we're hints for G/S remakes and Pokemon Heart Gold & Soul Silver we're made, there might be some in Pokemon Black & White too. However, that's just my theory, what do you think? Also, if you've spotted any more please post them in the comments and I will add to the list, thanks!
(I am aware that Pokemon HG/SS has been released US/UK but this is just a speculation)