This what I have learned :
digital inputs and outputs
Analog inputs and outputs
Line follower ir sensor
Pir sensor
Ultransonic sensor
Ir transmitter and reciever
Lcd
Key pad
Dc motor
Stepper motor
Servo motor
Interrupts
Timers
Watchdogs
Arduino with raspberry pi
This topics a made with it some projects so the question is what is next to learn about arduino ? What should be my next steps ??
You say you have learned these things but were you using a motor shield for the DC motor? How's about building your own H-bridge. And how many types of stepping motors do you now know how to control? What about micro stepping? And what about graphical LCDs. Do you know how to make custom characters?
As your list is hardware-centric, I would next task you to learn how to write in C++ and create a library for others to use. Learn what it means to abstract the hardware down to a few simple function calls. How to create a class which contains an Arduino library for, say, the graphical features of a particular LCD.
I would get myself familiar with low power, sleeping modes, running on batteries.
Also connect others devices using SPI and I2C to familiarize yourself.
Finally give a go on different communication methods... over serial, ethernet, wireless, can bus, etc. No much point in having stand-alone projects that are isolated or always connected to an USB port. IMO the fun begins when your projects start to talk to each other and to the Internet.
Cheers
Combine
I see a lot of loose "projects", but try to fit (as much as possible) into one single project. Make a line following robot, that balances, has WiFi communication and a display.
Putting all the different parts together may create whole new problems/difficulties.
Stop using Arduino?
Find another IDE, so it's easier to manage a lot more code. Learn how to set up a toolchain/compiler.
Try programming an ATMega328P without Arduino.
Why? Because it'll learn you how to other devices work, if you're comfortable programming an AVR without Arduino, it's just a little step to using PIC microcontrollers.
Hardware
Design yourself a PCB specific for the project you want to do.
Work on "Professional skills"
Try to make some actual designs, requirement analyses and make a report on the project. This way you can practice for school/future jobs' professional skills.
So what project should you do?
Something you would really like to do. It's as simple as that. Something of which you think is going to be a challenge (but not too challenging).
The Nrf24L01 is an inexpensive wireless transceiver with very good Arduino library support, and there are still library opportunities, like star and mesh interconnections. It also requires less overhead than WiFi and ethernet. If you're PhD material, you could implement the OSI stack.
Definitely try to make practical projects that make sense and are actually useful in real life. Automation projects for "smarter home" lets say really do come in handy and it is extremely satisfying when you use them. They don't have to be huge, just do something small that you find interesting. Documenting what you have done and keeping something like a report will help you big time in the future. I suggest you decide if you like C++ and if you do dig deeper into it.
