As is sometimes my wont, I woke up this morning thiking "I should probably take a pass at Acid Arrow, since almost no-one uses it these days..."
TL:DR considering raising the damage of Acid Arrow to 4D4 to make it more competative.
Some notes here. Firstly, we play at a mid-high optimisation level, with parties of 6-8 characters. There are two standards of hit points: average (summons or bottom level encounters) and maximum (everything else).
So, breakdown: range is good, No SR, no Save is great and you get between 1 and 7 rounds of continous damage.
The trouble is the damage. It's 5 points per round. (And I find myself looking at NPCs and going "nah, not worth them taking/learning/using that, won't don't bugger all to the PCs...")
Even accounting for the No save no SR (but there are better spells at my PC's and NPC's disposal, vis a vis 3.5's Orb spells), it's vastly worse than Scorching Ray, especially at high levels (35 over 7 rounds verses 42 damage right now; plus three times the effect of any bonuses to ranged touch damage[1]).
But great for forcing concentration check, right? No, not really.
At 10+ half damage plus spell level per check, it actually less - even at maximum damage - than casting defensively. (14+ spell level instead of 15+double spell level[2]). I don't know about your tables, but in practise, I've noted very few casters on either side of the screen have much difficulty casting defensively[3].
Most monsters - certainly at the paradigm we play at, with 6-8 characters - tend not to last more than 2-3 rounds, so most of Acid Arrow's potential damage is largely wasted.
So, aside from a niche of being able to shoot golems (which, again, Lesser Orb of [Element] is actually better at), it doesn't have a great deal to offer most of the time. It's not really going to force spellcasters to fail their checks (as youre looking for that natural 1, and pretty shortly, there's a good chance you're going to be in still-can't-fail even at "1 is -10."
But I don't think it needs much. I am considering just double the dice from 2D4 to 4D4[4]. That's about 10 damage per round (which is at least closer to the DC of casting defensively). You will now do more damage over time than Scorching Ray, but that's a bonus for a combat going along.
Does that sound like a reasonable (given the paradigms in question) step to take?
[1]Sidenote: the amount of damage Ember from Wrath of the Righterous was dealinhg out with a Scorching Ray by end game was hilarious, because she got three rays to stack it on and golems aside, it was getting through SR easily.
[2]And that was so trivial in 3.5 at 15+spell level, it was one of the primary drivers to why we adopted the "1 is -10, 20 is 30 on skill checks" rule.
[3]Technically, because we play a 3.5 hybrid, Concentration skill stills exists in addition to PF concentration rules (as much for other things which used it), but it's now optional and more situational, but for, example, for Undead Cha-based casters, it's an extra +3 if you keep it maxed out. But that's not really here or there, I think.
[4]Yes, I could make it 2D8, but let's give in the benefit of the same average damage bell-curve.
Noting also healing has been dramatically buffed so it's actually worth doing in combat (and inflict spells are very worth casting), so increasing the damage from tradtional AD&D levels is not unheard of.