From the arguements so far it appears that there is a worry of
musket troops appearing as soon as gunpowder appears. In the real world
you have to remember that it was over 200 years between the appearance
of the cannon and the effective use of musket troops.
The first documented use of a cannon by English troops
is around 1415, the use of the musket didn't really take off until
early 1600's and then most of the musketeers had to be defended by pikes
to stop the cavalry ripping them to shreds. Musket troops didn't become
all powerful until the 1700's when the bayonet was invented so the
musket troops were able to defend themselves.

Technological advances that possible go in between are the
better casting of cannon barrels, evolution of cattle design from
massive multiwalled affairs to low in the ground symmetrical affairs
with multiple gun ports and low in the ground, improvement of shot,
stabilising the formula so as to cause less powder explosions. To make
muskets there will be many more technological advances needed including
evolution in the whole concept of steel production, miniturisation
techniques, ballistic knowledge to improve shot etc.

If you are going to introduce gunpowder start with bombards and
maybe evolve into guns slowly. Why not try a gun which fires a quarrel
rather than shot, this would seem an area for alternate evolution, or
try gunpowder ballistas instead of a ball cannon. This would have
advantages in naval warfare as well as field warfare.

Don't let us old conservative roleplayers get in the way of your
imagination. Personally I wouldn't allow gunpowder. Nick