Cluster Bombs ("Math-Riotka").

Only small parts explode. The bigger ones just separate to the smaller parts. There is a delay between the separating explosions.

Hurting Moving.

Damage per second (-HP/sec), if you move under the negative effect (it is like Bleeding, if you don't stand on one place). Must be shown to the person, who was hit by this weapon (special icon on the HUD).

New tool: Speculum.

It is a special tool that makes your cosmetic item being mirrored (left <--> right).
Additional (prefix-)style (or - better word - version) "Mirrored" will be added to the item (so you can switch between normal and mirrored versions of the item). If the item has the styles, the version and the style can be combined.
For example, if you make the Mirrored Googly Gazer, you will have it for the left eye (the reflexion in the plane, which parts the body vertically - to the left and to the right). Also, the styles of the Googly Gazer can be switched too.

New tool: Rotator.

The hat can be rotated around the vertical axis. The minimum angle can be 15 degree (the option is:15, 30, 45...150, 165, 180 degree - clockwise or anticlockwise).
Special string can be added: "Angle: XX (CW/CCW)"(only if the tool has been used), like the "Paint: Y" or "Style: Z".

Sleep, my darling Sentry...

What if the Sandman's ball attract the attention of the enemy's sentry for a couple of seconds?
Throw the ball and run while the Sentry is shooting the ball.
Remember: don't run between the ball and the Sentry - because it chooses the closest enemy.

Idea for Minigun.

The spreading decreases after the 1st hit or kill (?plus xN speed of shooting ?)

Idea for Medigun

The closer to the mate, the faster it heals. Or vice versa.
Just an idea.

"Sentries's Fault".

Disguise Kit that attract Sentries' attention to you, though they don't shoot.
Maybe, it works only few seconds.

Just an idea, needs more thinking.

/to be continued.../

Different Trajectory for the Pipes.

The old one is /\. The one I suggest is \/, i.e. it goes down, firstly, then it flies up.
New grenade launcher, probably.