I've been thinking about how to fix the card system as well, but the best idea I can think of is marrying the utility and damage cards or separating them. One thing I know for sure though, is that the system could be a lot more creative than what they gave for DT. Here's a draft of how I would do a card system that allows thinking and optimisation:

Balance: 10% damage dealt up, 10% damage received down [ Straightforward, can use on anyone]
Bole: 20% damage received down, adds a portion of active mitigation and defensive stats into damage [The tank card]
Arrow: Increased autoattack speed and causes all autoattacks to hit twice [Best on jobs that already autoattack fast]
Spear: Increase crit rate by 15%, whenever a weaponskill/spell is used, apply a stack of Fury, Fury becomes Retribution at 3 stacks, Retribution causes the next weaponskill/spell to echo for 50% potency [Best on SAM when they use Meikyo or on PLD that's just starting their Blades combo]
Spire: Lasts 30s, increases move speed and allows every weaponskill/spell to grant 1 stack, every stack increases damage dealt by 2% to a maximum of 20% [Best used 15s before raid buffs so they have full ramp up in buffs]
Ewer: Restores 2000MP, every spell now costs 50% more but is 20% more potent (includes healing spells) [Basically usable on any caster except a BLM]

With that, you have a card system that has plenty of thought required to optimise fully while also having a diverse range of effects without having any cards that's just straight up useless. Also allows you to place cards on more than just the DPS jobs.