A good question.
It depends on the direction Crate wants the set to stay. I, for one, do not like Blade Arc and even if it did a gazillion damage I’d probably still hate sets built around it ( I have never played Bloodrager… ), but I am not of the mind that adding Physical Damage to BA is the call (for various reasons, one of which you just learned. 
) Those small values of Physical Damage pale in comparison to what one could theoretically get out of BA’s % Weapon Damage; I say theoretically, however, because Spellscourge uniformly has godawful flat Weapon Damage on account of its shield and Arcanist/Soldier offering literally nothing in that regard.
DoT Spellscourge BA might be able to squeeze out some damage with new Overload, however if you’ve played much Spellscourge you’d know the set is not actually all that tanky and losing the ability to benefit from ADCTH on account of going DoT isn’t great.
Furthermore, Spellscourge does have the energy regen to sustain BA + Maiven’s, but not much else. Devastation is a massive load to add in to the mix and for not entirely much gain. Other utility spells and a movement rune (which is also unaffected by -%Cost) will quickly kill your energy pool.
What this all comes down to is that Spellscourge needs:
- Damage
- Damage Mitigation OR sustain
- (More) Energy Regen
Given that that’s basically everything that every build needs, I’d personally argue that Spellscourge offers literally nothing to Battlemages and is not worth trying to build around. I don’t normally advocate for total overhauls (anymore), but the Soldier-side of Spellscourge sorely needs it.
If you had a place to put -% Armor, you’d also have a place to put -% RR. But we’re talking Battlemages, here. 
N Reduced Armor used to exist in the game but it was godawful (mostly because its values were too small (250ish range)) but Crate has since taken the stance not to reintroduce the stat.
Edit: Some ideas, just throwing them out there without many numbers:
- +% or +Flat Damage Absorption to Maiven’s (address Defensiveness)
- Redirect Spellscourge into IEE + OFF + Cadence/Deadly Momentum (solve Damage but compete directly with Warborn)
- I literally don’t know how I’d fix the Energy problem. I’ve been suggesting blanket reductions to Energy Costs on gear skills for a while now and while the last major patch did give us a lot of progress in that regard (thank you!) there’s still a lot of sore areas. Spellscourge has a lot of gear skills it’d like to use (Obliteration, Bone Lance, etc.) but it does not have the regeneration to sustain them, and by a very large margin.
- A fun idea might be to take Spellscourge away from Physical and do something truly unique, like Fire/Aether Counterstrike. Give it a Pox-like (or even CoF-like) skill that increases enemy speed but reduces their resists and damage, and that could really start to work in conjunction with Overguard, Menhir’s, and Devastation. Then go the Pyran route and give it something to use more actively, even if once every second or so.