Pokemon Renegade Platinum by Drayano reworks Platinum with significantly harder trainers, every Pokemon from Generations 1 through 4 catchable before the Elite Four, and expanded move and ability availability. Trainer teams are fully evolved and competitively tuned with held items that reflect post-Platinum competitive knowledge.
The difficulty increase is substantial but fair. Gym leaders and rivals use full six-Pokemon teams with appropriate items and strategy. Pokemon that required trading to evolve have been given alternate evolution methods that do not need a second DS.
Renegade Platinum is often recommended as a model for how a difficulty hack should be done. It requires a Nintendo DS with a flashcart, or any NDS emulator. Documentation packaged with the hack explains every change in detail.
