However, it fills no gap in the AH2 line up, imo. None.

We have ZERO turretless GVs with tank-caliber weaponry. That seems a very large gap to me, considering how much they were used.
At this time, I dont think we need another tank. If HTC adds one, I hope it is a Sherman M4A1/75mm or M4A3/76mm. But I think the gap that needs to be filled is the TD gap, imo. We dont have a high speed/med-high firepower/low armor vehicle and the M18 would provide for just that. Or, the M10 would be an easy add on becuase iirc, the chassis is the same as an M4 which we already have and all that would need to be modelled would be the new turret, new gun, and new ammo. The hull is the same as our M4 Sherman Firefly (correct me if I am wrong).
I can understand the desire for the M18, but it baffles me why people would want the M10 over so many other vehicles. It's armor, speed, and firepower are nearly exactly that of a 76mm armed Sherman.
There is your vehicle that truly fills no gaps in the AH line up.
Oh, I forgot, it would be designated a TD instead of a tank. Well, if it waddles, swims, and quacks . . .
The "classic" TD is a turretless tank like the already mentioned Stg's, Jadgpanzers, Hetzer (my personal favorite), and Russian SUs and JSUs. US TDs are TDs by classification, but differ very little from true tanks in form. This IMO makes the Stg's et. al. much more desireable.