While I do like Platinum, I don't think it does the 4th generation any justice. (I really just want an HD remake for Gen 4, really)
You know what I'd love them to do in the Gen 4 remake? (If they ever make that), go for something along the lines of ORAS and the Delta Episode.
Rework the plot, and make Palkia/Dialga actually begin to cause chaos in the Sinnoh region. Rip space open. Slow down time in some places. (In their respective games, of course)
Afterwards, make international Pokemon migrate to Sinnoh because of the anomaly.
And then in the post-game, make it so that the actions of Palkia/Dialga disturb the balance of nature, and Giratina is forced to act up and start "fixing" all of it. Make him not succeed.
And finally, as the greatest post-game content ever, make Arceus involved. You don't even have to give him a Mega or anything. Just give the players the glorious feeling of fighting the Pokemon God himself.
It would be the best.