Diep.io is a game sort of like agar.io except with "tanks". Instead just having being a cell, the player is a tank with a gun. Diep.io has more RPG-like elements. As you shoot things, you can choose in what way you level up. For example, you can choose between faster bullets or more hitpoints. Also, if you press TAB on the start page, you can switch modes including a domination where there are two teams that try to dominate the map, by capturing all the capture points.

This game is very fun in my opinion, but I still prefer agar.io personally. Mostly because I'm good at agar and I'm not good at diep

There is also Slither.io which is very very close to agar.io only (I think) a little bit better. Though, I can never play it because my internet is soooo stinkin' slooowww.

If you play team mode in diep, then you can just hang out in your teams side of the board until you are fully levelled which makes it easier.