Melf's Acid Arrow isn't usually a go-to spell in 5e, and it's even worse in BG3. Chromatic Orb: Acid and Arrows of Acid already do a similar thing at a lesser resource cost and have other effects. An upcast Chromatic Orb may do less damage after Acid Arrow's secondary damage is applied or when considering Chromatic Orb doesn't deal half damage on a miss, but being able to choose an element to avoid resistance/immunity or take advantage of vulnerability just makes it the better option 90% of the time. Arrows of Acid and Acid Vials are readily available and there's bound to be someone in a party with decent dex and a bow or strength for throw. And each of these create an acid surface, lessening the appeal of learning Melf's Acid Arrow and spending a level 2 spell slot. There's also the Acid Splash cantrip, which at level 11 does slightly more damage than base level Melf's Acid Arrow initial damage but with the advantage of being AOE. 

This mod makes 2 simple changes:

• The secondary (delayed) damage becomes AOE. The area is the same as the acid surface created. A Dex save is made to resist the dot and the acid condition. It will harm allies, but is still an Evocation spell so Sculpt Spell helps. 
• Upcasting damage is better on the initial target. Upcasting is now 2d4+1d4(delayed AOE) per level beyond 2nd (from 1d4+1d4 delayed). 
  

I'm calling this balanced and as close to the original spell while maintaining it's niche uses. It now does slightly less damage than Chromatic Orb on a hit cast at the same levels but the secondary damage does finally make sticking with one element worth it over the damage versatility of Chromatic Orb. The AOE targets take about half as much damage compared to Shatter at the same level but Melf's Acid Arrow targets are more likely to fail their save. And it's all still much lower than what Fireball can do. I'd almost say this is still too weak, but acid damage is reliable since there is neither much resistance/immunity or vulnerability, and there is a set of gloves that improve damage.

Install using BG3 mod manager. Should work on existing saves without the need to respec. Can uninstall safely without breaking save files.

Compatibility: Shouldn't conflict with anything unless it changes the Melf's Acid Arrow spell. Conditions and surfaces are not changed. If using other mods that alter Melf's Acid Arrow or multiple spells, loading this mod after (below) those may help compatibility but no guarantees.

See my other spell mods:

• Infestation - 5e cantrip
• Better Flame Blade Synergy
• Poison Spray is Cone
  

References and special thanks to:

• Let's Play with Fire Wiki
• Modding a custom class in 10 minutes or less
• Larian Export (LSLib) Tool
• BG3 Mini Tool
• Baldur's Gate 3 Modder's Multitool
• D&D 5e
• My TTRPG friends