Increase jumpheight while holding down a key

In order to achieve this, my approach would be to use a timer and a max time.

First, change line 25 to:

 if (Input.GetButton("Fire1") && isGrounded == false && j != 0)

Next, lets add some variables at the top of your script. Something like:

 public float jumpTime, maxJumpTime;

Now lets set the jumpTimer value to zero if we just jumped off the ground. To do this, lets add a statement 18 such as:

 jumpTimer = 0;

Finally, lets apply the jumpTimer in your DoubleJump method:

     public void DoubleJump()
     {
         if (jumpTime >= maxJumpTime) return; //Return if we have already hit the max time.
         jumpTime += 1 * Time.deltaTime; //Add jump velocity each second.
         rigidbody2D.AddForce(transform.up * jumpTime,ForceMode.Acceleration));
     }

As far as the gravity thing - that's probably best handled by another script. Are you saying that you want gravity to become stronger the higher he goes? Is there a terminal velocity that you'd like to reach? Rigidbodies usually do a pretty good job with physics.

hello I have this code for you, I added the comments that I would have typed into this so will not say much but feel free to ask also I changed OnTriggerEnter2D to OnCollisionEnter2D so I could make a quick prototype without needing custom collision logic you may need to change this back for yours

using UnityEngine; using System.Collections;

public class IncreaseJumpHeightTest : MonoBehaviour {

 public float jumpPower;
 public float doubleJumppower;
 public int jumpCount;

 //this is how high realative to the player that the player can jump
 public float maxHieght;

 //this is the max hight in world space the player can jump
 private float targetHieght;

 private int j;
 
 public bool isGrounded = true;

 //when is jumping is true force will be added to the rigidbgody2D
 public bool isJumping = false;


 //reach hieght is V squared = U squared + 2as but with s as the subject

 /*
  * we make S the subject to make 
  * 
  * S = V squared - U Squared
  *     ---------------------
  *         2 * a
  * 
  * S = distance
  * V = final velocity
  * U = initial velocity
  * a = acceleration
  * 
  * this means that it will tell us how far we have to go to get to the Top of the jump
  * 
  */
 float ReachHieght()
 {
     //our final velocity at the top of the jump we want as zero
     float v = 0;

     //our initial velocity is our upwards velocity this frame
     float u = rigidbody2D.velocity.y;

     //acceleration is defined in unity's project/physics settings
     float a = Physics.gravity.y;

     //S will tell us how far our current velocity will get us
     float s = (Mathf.Pow (v, 2) - Mathf.Pow (u, 2)) / (2 * a);

     return s;
 }

 void Update () {

     //if the distance that our current velocity will travle this frame + our current position
     //             is greater than or equal our target hight stop adding force
     if (ReachHieght () + transform.position.y >= targetHieght) 
     {
         isJumping = false;
     }

     if (Input.GetButtonDown ("Fire1") && isGrounded == true && j != 0) 
     {    
         //set isJumping to true
         isJumping = true;

         //get a world position to reach
         targetHieght = maxHieght + transform.position.y;


         Vector3 velo = rigidbody2D.velocity;
         velo.y = 0;
         rigidbody2D.velocity = velo;
         
         jump ();
         j = j - 1;
     }
     else if(Input.GetButtonUp("Fire1"))
     {
         //if player lets go of jump button stop adding force
         isJumping = false;
     }


     if (isJumping) 
     {
         //add jump force
         jump();
     }

     if (Input.GetButtonDown ("Fire1") && isGrounded == false && j != 0)
     {    
         Vector3 velo = rigidbody2D.velocity;
         velo.y = 0;
         rigidbody2D.velocity = velo;
         
         Doublejump ();
         j = j - 1;
     }


 }
 
 void jump (){
     rigidbody2D.AddForce (transform.up * jumpPower);
     Debug.Log("Jumped");
 }
 void Doublejump (){
     rigidbody2D.AddForce (transform.up * doubleJumppower);
 }    
 void OnCollisionEnter2D(Collision2D other) {

     Debug.Log("Grounded");

     if(other.collider.tag == "Ground")
     {    
         j = jumpCount;
         isGrounded = true;
     }
     else 
     {
         isGrounded = true;
     }
 }

}

one limitation of this is if your initial force you add is too great it will launch the player above the target height some code to add a counter force to slow it down in time would be needed if you wanted a really quick launch.