There's a lot of hate for the PS2 games now. It's kind of funny that a while back it was the N64 games that were getting put down. Iga even apologized the N64 games were ever made as a result of the backlash and took the series in a different direction for 3D. So far he hasn't been able to bring the series to 3D successfully either though. To find out why, ask yourself what made Symphony of the Night - the most critically and commercially successful entry for the series - work.
Story - Alucard was re-introduced as a badass. While the story was delivered cheesily, the premise was deep - Save mankind by kicking the ass of the most evil person on the planet, who just happens to be your dad.
Gameplay - Despite all of the options, the gameplay was very simple. One button attacked, one button used magic, and one button jumped.
Variety - Once the basic gameplay was established, weapon and magic customization opened up options to provide a level of depth for players who wanted to tinker with their character.
Level Design - The satisfaction of leveling up your character would not have been possible if it weren't for the way that items slowly became attainable for you by reaching new areas as the game progressed.
Obviously, they have tried most of these in 3D, but it has not worked, so we're back to square one. They need to back to the drawing board and look at what works and what doesn't. Here are some of my ideas.
Story - I don't really think story has been the problem for the PS2 games, I just think they could flow a little better. Not a whole lot needs to be done here. Story is by far the biggest asset Igarashi brings to the series.
Gameplay - Go back to the basics and do more with less. I say make a Zelda clone but instead of a sword, you have a whip. The point is have very simple controls, just like SoTN and the 2D games. Have one button for your whip, one for your sub weapon, one button for magic and one button to jump. One shoulder button should also be used to lock-on and block.
Once this basic gameplay mechanic has been established, other elements can be added on like combos. Like Zelda, pointing your control stick in different directions could cause different effects (ex. while locked onto an enemy pull back on the control stick while swinging your whip to throw the enemy in the air. From there, you can attack him in the air and throw him to the ground. Once on the ground you can then use holy water to finish them off.)
The point is, your whip should be your primary weapon and they should focus on keeping it simple. Imagine if an enemy gets too close, you let go the lock-on button and mash and hold the attack button. This opens up a new combo sequence and you grab onto the enemy. Once you grab them you can then beat them with the butt of your whip, your cross, daggers or holy water.
Your whip could also be used to take down bigger enemies or flying enemies. Imagine if your fighting a boss. You lock onto his face and once you attack him enough, be bends over to rub the blood off of his eyes exposing the horns on top of his head. You then lock onto the horns with your whip. Once your whip is attached, you pull back on the controller and pull him to the ground. From there you are automatically put on top of him and have to tap the attack button repeatedly to finally kill the beast. That's one example, perhaps you would pull his feet out from under him, or his hand into a meat grinder instead. The point is to keep the gameplay simple.
Variety - The idea of leveling up could be collecting materials (herbs/chemicals/metals/money) that you could then take to a blacksmith/alchemist/shop and fuse the items to your whip/armor to make it more powerful. Some items should come from fallen enemies (ex. all enemies drop herbs/meat which will heal you. They also drop daggers which you throw for long distance attacks. However, a fire/ice enemy will occasionally drop fire/ice essence which can be fused to your dagger to cause more damage to certain enemies.) and bigger items should be found throughout the levels.
Level Design - The jump button is there for a reason - use it. Your whip can also attach onto things - use it as well. Think Tomb Raider, Prince of Persia or Bionic Commando for platforming/swinging. Your whip can be used to pull switches and solve other puzzles as well as platforming. Items like bigger bags for carrying more holy water or daggers should be hidden throughout the stage. Power-Ups like double jump boots should be received once you beat a boss.