The main "problem" with the way SE has designed Thief is not the lack of DD abilities or their vision of the job. The "problem" is that SE made Treasure Hunter a passive trait that's almost entirely independent of Thief skill and dedication. Over the past few updates, SE has been back-tracking and trying to make it somewhat skill dependent with their goofy proc system, but autoattacking/using SA/TA is hardly "making it skill dependent" at least not to the extent that skill/gear determines DD potential.
If I was going to change it, from a design standpoint, I'd eliminate the current multi-tiered job trait TH system (though I'd keep the tiers) and instead make it dependent on Thief's strong point, spike damage. You borrow the skill/gear dependence of DDing by making it depend on damage. Set a TH effect system with multiple tiers (instead of the three current tiers) with /THF being capped to Level 2, that works like this:
Level 1 +(p-1)*p) : Thief is on the hate list
Level 2 +(p-1)^2*p) : Do >5% HP damage in a single action or >2*Monster DEF, whichever is lower
Level 3 +(p-1)^3*p) : Do >10% HP damage in a single action or > 4*Monster DEF, whichever is lower
Level 4 +(p-1)^4*p) : Do >15% HP damage in a single action or > 6*Monster DEF, whichever is lower
Level 5 +(p-1)^5*p) : Do >20% HP damage in a single action or > 8*Monster DEF, whichever is lower
Level 6 +(p-1)^6*p) : Do >25% HP damage in a single action or > 10*Monster DEF, whichever is lower
Level 7 +(p-1)^7*p) : Do >30% HP damage in a single action or > 12*Monster DEF, whichever is lower
etc.
Where p = (Base Drop rate + TH Gear/100)
This kind of system, if properly balanced (and it's possible the numbers above are not), would force parties to cooperate with THF game mechanics if they want drops. Not only that, but high tiers of TH would only be obtainable by THFs with considerable dedication to the job and a little luck. The highest tiers of TH would be unobtainable on anything notable. And most importantly, if they change the system to something like this, they need to TELL US EXACTLY HOW IT WORKS.
By this system, a 10% drop rate with Level 5 TH would become a 57% drop rate for THF main (TH+3), a 38% drop rate for a less dedicated THF (Level 3 TH and TH+1), or a 27% drop rate for /THF. The above math is actually just an extension of the way the game's TH trait system currently works, though it's unclear how the current "level" system works. Point is, the above system would make the whole party revolve around helping the Thief get epeen damage shots.