I don't know if TA can introduce GWM to Solasta or not. To my understanding, due to license and copyright issues, TA wouldn't be able to implement GWM as it is in Solasta. GWM is PHB content. However, for weapons like greatsword or greataxe, GWM is necessary to further increase their damage potential. So, any alternative feat(s) for great weapons are needed, from modders and/or TA. Right now, without GWM, the damage effectiveness of any greatweapons is very limited.
GWM as of now is:
1) On your turn, when you score a critical hit with a melee weapon or reduce a creature to 0 hit points with one, you can make one melee weapon attack as a bonus action.
2) Before you make a melee attack with a heavy weapon that you are proficient with, you can choose to take a -5 penalty to the attack roll. If the attack hits, you add +10 to the attack’s damage.
What would be the best alternative option?
I think since the Sylvan archer feat of Solasta contains a part of Sharpshooter feat, (Your ranged weapon attacks ignore half cover and three-quarters cover) I guess homebrewed alternative of GWM may able to follow the same procedure: Just contain the first part of the feat description "when you score a critical hit with a melee weapon or reduce a creature to 0 hit points with one, you can make one melee weapon attack as a bonus action." part, and ignore power attack option and increase +1 Strength. It followed the same approach with Sylvan Archer feat.
Or, we can go something truly creative. The key to the current GWM is a bonus action upon condition. For homebrewed ones:
It might be able to change the condition. Instead of inflicting a critical hit or killing the target, maybe just use "every 3rd hit".
Or, it might be able to change the bonus. Instead of bonus action, just increase the x amount of damage for y turns from the next turns.
What would be the best alternative for GWM?