Wave 5 things get easier in the figure department. With season 3 you can do the Ninja Rangers (Auto-Morphin?). For villains Rito, Tenga Warrior, and a monster of the day. Vampirus already got a 5" figure from Bandai, so I'd hope Playmates does something completely new like Crabby Cabbie. Add Ninja Zords, a large scale Ninjor, a new Morpher with Nina Coins, and three Shark Cycles.

Wave 6 you can do Metallic Armor Rangers, which would be Kat's introduction to the line. For villains, Master Vile and a monster of the day (Blue Globber would make a lot of sense). I only included two villains because if we're assuming each wave is capped at 9 figures, I'd use that final slot here for a 5" Ninjor. Round things out with the Shogun Zords, the remaining Shark Cycles, and any remaining Power Weapons.

Wave 7 probably wouldn't happen, but there's enough in the Alien Ranger saga to support a wave. For the basic figure wave there's the 5 Alien Rangers, Hydro Hog, and I could see them doing three kid Ranger two-packs similar to how they do their kid turtles to round things out. There's the Battle Borgs for Zords, the Alien Ranger sword and sidearm for roleplay. No vehicles come to mind though.