Seconding this and thirding Advance Wars.Elijin said:Front Mission! Always Front Mission.
For the former, a return to FM4's turn based strategy with perhaps a touch more flexibility and action (for example, I saw a tactical war game similar to it once called Ring of Red where enemies and allies only moved when you did. That could be used to blend turn-based mech combat and action). I even wrote a script for it up to about the 8th mission. For the latter, I kept expecting to see a new game come out in the 'Days of Ruin' timeline linking it to the original trilogy by having Tabitha and Waylon found Black Hole and Will & Co. found the other nations.
The Lost Vikings. Blizzard apparently cares enough to give it cameos and references in both Warcraft and Starcraft as well as hire Gilbert Gottfried and Jim Cummings to voice the PC remake, but not enough to ever give us a third game. Probably because they know there would be pressure to make it 3D, which might not work with its style of puzzle-platforming.
Ninja Gaiden. I was just thinking about this the other day when someone made a thread asking about old games that I liked without nostalgia goggles. Not that I felt the first of the new trilogy was bad, just that it kind of lost its way after, losing the difficulty it was known for in the third game and embracing Call of Duty-style storytelling and enemies of all things. Like the mystical lineage of the Dragon Clan was meant to fight off terrorists instead of demons and the Spider Clan. Maybe we could introduce some other clans sworn to that duty in a new game, like the Eagle and Bear clans specialising in acrobatics and strength respectively. Regardless of the story, it's important to face Ryu with brutally fast and powerful ranks of murderous enemies both human and non, and make him feel even greater than them upon a well-earned victory.
Demon's Crest/Gargoyle's Quest, with Firebrand as flying demon Kratos who can also change to various elemental forms. Is Firebrand the first side character to ever get their own spin-off games? I think might be. He predates both Zero and Raiden.