A few ideas

Juggle The Factory but instead of making the horizontal carry from left to right or right to left, carry the ball from front to back & drop the ball down behind your shoulder for a Behind the back catch.

Juggle Eriksson's Shuffle but instead of holding a ball in the hand catch & throw it vertically in a Fork. This is a shape distortion of The Box.

Hold all 3 balls in one hand & extend your arm out to the side. Throw a Triplex in towards you so that all 3 balls form a triangle in line with your body with 1 ball at the bottom & 2 at the top. Claw the lowest ball with the same hand that threw it. Then with the same hand catch the ball furthest away from you Penguin style. At the same time catch the ball closest to you behind the back with the other hand. Throw a Split multiplex behind the back

Juggle The Factory or Relf's Factory then every now & then pause the pattern by catching the dropped ball with an Elbow catch (the hard way of course).

Catch one ball in an Elbow catch (the easy way so the arm is bent) reach behind the head with the same arm then drop the ball held in that hand behind the opposite shoulder. At the same time throw the ball in the other hand up to the hand behind the head for a Behind the head catch before quickly reaching back to catch the ball that was dropped over the shoulder.

Singapore Shuffle with all the throws made Under the leg & all the non claw catches also made Under the leg. This one is pretty energetic.

Throw a Triplex Behind the back, Claw the furthest ball, catch the lowest with an Elbow catch (the hard way) & catch the last ball in a Shoulder trap all on the same side of the body.

Three in one hand but make all catches with both hands, just like when you first learnt to catch as a kid. This is really difficult!

Juggle The Factory but make the horizontal carry first, then make the vertical carry while your arms are crossed over.

(4x,4x)(4x,[4x4x])* (5 balls) which in English is a 4 ball wimpy pattern with a multiplex throw.

531 with the 3 thrown Behind the back & the 1 Handed behind the back - saw this in Bremen, very nice.

531 with the 5 thrown to height 3 then knocked back as a Body bounce, bouncing off the knees is an easy one to start off with but outside of the foot looks much better.

33441 with the first 3 thrown straight up on the inside of the incoming ball (like Two in one hand rolling out) & then caught as an Under the arm catch.

504 with the 4s thrown Under the arm.

531 with the 3 caught Behind the head.

423 while Fork catching the 4s, throwing & then Penguin catching the 2s.

Mills Mess with the Over the top ball thrown Over the shoulder & the 'U ball' caught Penguin style (tip - stand up straight, don't lean forward).