Is there a way to upgrade the combo?

[Ultra]

In the original hammerwatch game you could buy upgrades to your combo. Im wondering if that is possible in this game or if all upgrades to the combo is only related to specific items

[Cirno]

There are a few items to boost your regen/damage etc during the combo.

[SpinDashMaster]

It's tied to items. the only ability that seems to come close is Thief's Slice and Dice, which is a stacking attack speed buff per-kill. Note, however, that this ability is not actually tied to Combo.