If you turn them into something like Soldados then they’ll compete with Musketeers. Either they’ll be worse and Grenadiers won’t get used or they’ll be better and Musketeers won’t get used. Neither option is good.
The issue is how the role of Grenadiers evolved over time from just a guy that lobbed grenades to elite heavy infantry shock troops. The depiction of Grenadiers is as the former, but they have the heavy infantry tag as if they were the latter. So they end up defying all the other roles of heavy infantry by being effective against light infantry and weak against cavalry.
What I think needs to be done is to drop the heavy infantry tag and add a new “siege infantry” tag for all infantry currently tagged as siege units.
Siege infantry would occupy a role that falls somewhere between infantry and artillery. Essential, siege infantry would be artillery what shock infantry is to cavalry. All siege infantry would share high ranged resistance, reduced damage from building fire, high siege attack, and a weakness to cavalry. They could have varied specializations just like artillery.
Siege Infantry
- Siege infantry does x0.75 vs cavalry
- Buildings do x0.5 vs siege infantry
- Artillery does x0.75 vs siege infantry (except culvs)
- Culverins do x3 vs siege infantry
- Light cavalry does x1.5 vs siege infantry at range
Anti-Infantry (like Falconets)
The grenadier units could have their base attack toned down a little and have a multiplier vs infantry to compensate.
- Grenadier
- Fire Thrower
- Chakram Thrower
- Flamethrower
Anti-Artillery (like Culverins)
- Huaraca
- Arrow Knight
- Humbaraci
Anti-Building (like Mortars)
- Ram
- Petard
Generalist
- Mantlet
- Huron Mantlet
To represent the shifting role of Grenadiers over time, the Line Grenadiers card could transform them into something more like Soldados or Giant Grenadiers and give them the heavy infantry tag. This could be limited to only select civs to prevent civs without Musketeers from getting an equivalent.