What kind of game should I design

Always. It has so many fun concepts and you can start small. Make it 3d with a lot of neon lights. You can keep fleshing it out and it will keep you busy for a year.

I am amateur in game designing

Since you’re remaking it, the design is already, kind of, there but do not let that stop you from introducing new elements and play with the existing ones. Everybody likes a boss battle! With UE4 an old arcade game can look and feel striking.


TPP Shooting game but I made that game
with the help of tutorial

So is Space Invaders really, but the board is fixed (does not have to be if you go wild). There will be overlapping functionality like camera controls and shooting and new ones, like spawning enemies in patterns. A great way to learn program a game loop.

could I get a mentor to help me to
design a game

Try here:
https://forums.unrealengine.com/community/got-skills-looking-for-talent/job-offerings

But do have a look at the Sticky before posting.