Host Demi-Plane

By studying for a multitude of years (or not i.e. sorcerers) you can put all your arcane knowledge into a single spell to make your own Demi-Plane. The portal will last for as long as you have concentration. To open a portal you in combat you can start making the portal at the start of your turn and become exhausted as a result you will have no more movement and actions the rest of your turn. A creature has to be willing to enter the Demi-Plane.

How the portal manifests is dependent on the character and their personality, for example a clown would manifest a carnival gateway. And how the demi-plane itself looks is determined the same way (a fire giant plane would be a volcano, an artificers might be a laboratory they can study in.)