I didn't use a lot of scrolls in Deadfire so I don't know a lot about their workings by heart tbh. But first of all: including at least one class that gets the highest Arcana bonus might be good? The Wizard gets +2 from the start and with Chamaeleon's Touch it's +3.
But if Shared Nightmares works with scrolls I'd also say a SC Cipher (for me it would be a Psion) should be great. Also Time Parasite and Reaping Knives might be awesome with scrolls. I mean those synergies won't click until the latest char levels - but you'll have a great caster character right away.
The Barb's appeal is stuff like Frenzy (Blood Frenzy's DoT on crit?), Bloodlust, Blood Thirst (and maybe even the +25% dmg from Blooded), right? Furyshaper to get even higher scroll speed with the Frenzy Ward and later health drain with the Blood Ward?
How about a Druid with Cat form? Should be able to cast scrolls very quickly, too. Advantage would be that it wouldn't require any lengthy setup. Just shift and activate cat flurry which is both instant casts without recovery. Can Shifters cast scrolls while shifted? I never tried... Does the PL bonus of Ancients (for beast spells) or Furies (for elements spells) work with fitting scrolls (do the scrolls like Avenging Storm for example carry the "elements" keyword?).
Paladins' Eternal Devotion adds its burning lash to spells etc., I suspect it also adds it to damaging scrolls (never tried)? If so this would be very good. Should the lash from Blightheart, soulbound to a Wizard, also add to scrolls and not only Wizard spells I think a Paladin/Wizard would be excellent: high Arcana bonus, double lash on scroll dmg... cannot test bc. I am on vacation in Latvia.
Do the elemental PEN abilities (Scion of Flame etc.) work with scrolls? If so all classes who have access to these abilities might be worth considering.
Are there scrolls that do piece, crush or slash dmg? Because that might trigger Deep Wounds of the Rogue maybe? Speaking of Rogue: what about the Debonaire's 100% crit conversion against charmed enemies? Does it apply to scrolls? That would be great. Debonaire/Beguiler or -Psion or Debonaire/Chanter could mass-charm and then drop a friendly fire scroll that mostly would crit I guess? Does the Assassin get anything out of scrolls when he casts them from visibility/stealth?
Lots of questions and little answers from me, sorry. 😉