Just what the title says. Instead of an always laggy tech to research, Arquebus should have been an imperial upgrade for the Portuguese hand cannon, possibly one available to their allies. Instead of ballistics, their shot should have twice the normal shot speed, and they should have 50 instead of 35 Hp.
This fills gameplay, civilization and historical niches.
If you really wanted, Piano cannons could have been moved to the Seige workshop and been reworked as well. I’d say slower and with a minimum range, maybe even with a packing/unpacking time, but made much more deadly.