How can you take a game like Contra or Abuse and make it more frantic? Let's first step back and look at a top-down shooter like SmashTV or Crimsonland. When zombies are coming at you from all sides, you really have to move and shoot, clearing a path for more moving and shooting. You can easily find yourself boxed in and become more and more frantic as you look for a path to clear and move through. That's just not possible in a game like Contra. The enemies can come at you from two sides, the side you're scrolling into, and above. You never really get that sense of, "I'm screwed, what can I do?". You're just holding the fire button and running, occasionally looking up to take out things above you. How can we create that same frantic feeling as an overhead shooter?

For a frantic feeling, reverse all characteristics that give the player some time to think:
-Enemies that appear usually have three phases: visible, vulnerable to your attacks but unable to attack you, close and engaged. Just shorten the first two phases.
-Instead of sparse and slow bullets, make the player dodge a lot of difficult hazards.
-Instead of letting the player clear safe portions of a level where he can rest, ensure that a patrol or some suppressive fire eventually goes everywhere.
-Vary threats to avoid habits, for example enemies can shoot with random parabolic shot trajectories, reaching the player from every direction, rather than always orizontally.
-Make the enemies move fast and the player move faster, emphasizing fast reactions and good overall maneuvering over good positioning and pixel by pixel advancement.

These changes obviously increase difficulty, so they should be balanced by other measures:
-Far less lethal damage that the usual standard of death at first touch. Getting hit a little might become part of successful strategies, like in Doom.
-Open environments with few serious obstacles. Very high jumps, harmless sliding, running up walls, ramps instead of steps, etc.
-Great and interesting firepower for the player, like bounching thrown weapons, bombs and mines, grenades, etc. along the lines of Worms and Liero.
-Modest requirements of precise movement (grabbing stuff, using stairs, slipping down ledges, jumping over pit traps, etc.) and precise timing (intermittent hazards, running through traps, etc.), and not too disastrous failures in case of slightly wrong moves (e.g. slowing down at the wrong point in Sonic).
-Flexible player movement; without effective choices and complex things to do good players would be as helpless as inexpert players and the game would be unfair and overwhelming, not frantic.

Can you explain what sort of game you have in mind? Contra and Abuse are different, and I find Abuse more frantic than traditional platformers like New Zealand Story or Ghosts'n'Goblins.

The Metal Slug series generally feels frantic in the last few missions. Some things I can think of that helped here:

* Enemies attacking from behind. If you're going into the enemy base solo, then it makes sense that you can't actually clear out an area entirely; thus enemies are always attacking you.

* Enemies hiding behind sandbags. Typically, you can't hit them unless you get in close; unfortunately, by doing that, you come into range of the next set of enemies hiding behind sandbags. This naturally "pulls" the player deeper into the level.

* Similar to the above, have various things drop bonuses which disappear if not picked up quickly. Even if the bonuses only provide points, they'll keep the player moving into range of more enemies.

* Rapid shifts of focus. Instead of continually walking forward, have the player occasionally have to find off helicopters from above (while not moving, and while having enemies leaping in from the sides, naturally), or crawlers bursting out of the earth, et cetera. In other words, every once in a while, pause the horizontal motion for a stationary miniboss of some kind, which can be precisely as frantic as you like.

Depending on your setting, you could try enemies that can form a "wall of flesh" even in the vertical direction of the side view, like zombies that can climb over each other, or enemies that spawn copies of themselves, build a web etc. Keeping the Abuse firing system will help I think, as you have to prioritize the whole 360 degree arc of fire, instead of easy/formulaic Contra-style strategies like firing mostly forward but every now and then turning to fire behind.

It'll play most like Abuse. 360 degree aiming, able to travel anywhere in the level (ie: backwards) and mostly confined to indoors. Unfortunately that rules out helicopters from above. ;) Or does it? I suppose a roof exploding and a helicopter flooding in from above would be rather interesting... but most certainly few and far between.

The game is going to be a mix of human and mutated targets (zombies, flying zombies, whatever). I definitely like the idea of sandbagged targets with parabolic shots, and lots of mutated enemies could be crawling along the ceiling.

It's hard to keep waves of bullets flying though -- you can either jump or duck, neither of which are terribly accurate for efficient dodging. Zombies coming straight for you are just going to be mowed down by your superior fire power and there's no way for them to overwhelm by surrounding you like an overhead shooter.

Add more trajectories that attacks can come in on. Straight-firing shots are pretty much either easy or impossible to dodge, but if you up the complexity then you have a smoother continuum of difficulty. Parabolic shots help; so do enemies that leap in (which you can thus shoot out of the air, or dodge; but in the latter case then there's an enemy close by).

One other thing to keep in mind is a form of "wall" pushing the player around. This doesn't have to be a literal wall, but the player should be kept on the move because of the mass of enemies on one side. Keeping the area in front should be reasonably straightforward, but only if you keep moving; if you turn to take down the guys behind you, then the other side gets to build up some momentum. Keep the pressure on and you'll make the game more frantic.