New vials that slightly increase one stat and slightly decrease the other, but they don't have "super" versions like the normal vials, here are the my ideas:
attack speed vial: attack speed increased, but lowers general damage done.
weapon damage: sword vial: + sword damage, - gun/bomb damage. gun vial: + gun damage, - sword/bomb damage, bomb vial: + bomb damage, - sword/gun damage.
walk speed vial: + walking speed, - defense.
defense vial: + defense, - walking speed.
charge time vial: - charge time needed to do the charged attack, - defense.
We have trinkets. And UVs. And armors with passive bonuses.
walk speed is useless, move faster to die faster? and move bonus arent that noticeable - see mercurial
Weapon damage ones don't actually have drawbacks. Bombers won't care for the reduced sword/gun, swordsmen won't care for lower bomb/gun, etc
For the rest...
Basically this can be great for new players on T1 since you cant get bonuses that early, but become useles the deeper you go since your gear can provide a permanent perfect bonus.