You need to decide how the combat rules should behave, then formulas that fit the requirements will be easy to find.
For example, consider critical hits:
- Why do you want higher damage variance? How is low variance bad? Barring strange effects, higher variance gives a chance to the weaker party, and every game needs to find the proper balance between confidence and uneasiness (for strong characters) and between despair and gambling (for weak characters).
- Wouldn't rolling virtual dice for damage, with the possibility of choosing the probability distribution of damage output quite freely for a desired expected value (in particular, larger variance can be obtained by summing fewer larger dice), be more flexible than a mixture of "high" and "low" damage?
- Are you OK with a bimodal distribution? Would the players notice?
Or consider special (magical) attack and defense:
- Against a given reference opponent (e.g. Str 50, Def 50, Mg 5), increasing Mg is equivalent to increasing Str and Def. What is the equivalence? Does it depend on your stat values? Or on the opponent's stat values?
- For example, you might want a very linear system in which a +k Str or Def increase is always as useful as +1 Mg, regardless of your stat values, and against any enemy: expected magical attack damage could be (Str+k*Mg)/(Def+k*Mg).
- Or you might want a strongly level-based system in which lower Mg values don't stand a chance against higher ones, with Str and Def mattering only in the same Mg tier: expected magical attack damage could be min( (Str-Def)*(attacker Mg - defender Mg), 0)
- How many points of Str, Def, or Mg should equipment be worth? Should it vary according to stat values?