Just buff Bloodspiller.

[SummerScorcher]

I would quite like:
Bloodspiller - additional effect: mimics the additional effect of the last used combo weaponskill.

So depending on whether you used Syphon Strike, Soul eater or Power slash before Bloodspiller you'd either restore some MP, restore some HP and get 10 blood, or generate additional enmity.

I like this idea, but wouldn't you always want to use Bloodspiller after Syphon Strike if this were the case? And, what if you were to use two Bloodspillers back to back? Would it just mimic the previous Bloodspiller's effect, or not do anything at all? This'd also bring back the DPS-neutral dillema, to which I would suggest to only have the additional effect apply on naturally generated Bloodspillers (though I admit, not sure exactly how they would do this).

[Jandor]

I like this idea, but wouldn't you always want to use Bloodspiller after Syphon Strike if this were the case? And, what if you were to use two Bloodspillers back to back? Would it just mimic the previous Bloodspiller's effect, or not do anything at all? This'd also bring back the DPS-neutral dillema, to which I would suggest to only have the additional effect apply on naturally generated Bloodspillers (though I admit, not sure exactly how they would do this)

Thinking about it, maybe BS as a combo 'replacer' would be better.

Additional effect: can replace combo action Syphon Strike, Souleater or Power Slash, duplicates the replaced actions additional effect(s).

So if you have 50+ blood, you can go Hard Slash - Bloodspiller - Souleater, BS in this case would restore MP.
Hard Slash - Syphon Strike - Bloodspiller would see BS generate 10 blood (and in Grit restore HP.)
Hard Slash - BS - BS would restore MP, then generate 10 blood and (in grit) restore HP.
Hard Slash itself can't be replaced, if you're not mid-combo Bloodspiller just works same as it does now.

IIRC Bloodspiller works out as a slight DPS decrease over a long enough fight because it delays MP restore and blood generation from your 1-2-3 combo. Letting it slot itself in there and keep the combo moving along would in theory "fix" this.

You're probably right that if it duplicated last additional effect everyone would just hold it for use after syphon, this way it doesn't generate anything extra, it just doesn't slow anything down either.