Where, oh where, are the wildcats based on the RUM case cut to 2.5 inches? While some of these would duplicate the Dakota and Imperial rounds, the RUM is a far better choice as the rim size matches the standard Mag both face exactly, while others do not.

The availability of actions for the WSM and SAUM case is very limited if one does not want to rework the bolt face, and there are lterally millions of '06 length mag actions out there.

Cutting .035 inches off the RUM (.24 from the 338 RUM) only reduces case capacity by 14% and velocity by 4.5% allowing us to duplicate the ballistics of such fine rounds as the 7MM STW, 300 Weatherby, 358 STA and 416 Rem Mag in a standard action with no changes to the bolt face. As Dakota has demonstrated, the 338 and 375 in this case size are fine cartridges
If one works with the shorter 338 RUM brass, the shoulder can be pushed back .24 inches and only the neck cut.

I'm a big fan of wildcatting, and wouldn't want to discourage anyone, but I do have at least one answer to your question, where are they.
The money to design and cut a new chamber and dies, etc., is quite a bit more than that for opening the boltface to .545", and you'd gain maybe 1-3gr of powder over Dakota, depending on body/shoulder design.
Myself, I'll take a nonrebated case over a rebated one.
There are other answers, I'm sure.
But at anyrate, I suspect we'll be seeing these pretty soon from somebody. Someone with a vendetta against Dakota, perhaps.

They are out there already. The Howell Series, I have a 416 Howell. Although the Howell's are based on 404 Jeff, BUT they are so close you can just about use them interchangably.

Nope, much more akin to Dakota.
If you want to say the RUM is the same as the Imperial (Canadian) Magnums, then, yeah, the Howell is the 2.5" RUM.
Rum is a wee fatter and has a rebated rim.
Only real difference in Howell and Dakota is that Dakota took out that .002" rebate from the Jeffery case.
I doubt you'd see any real performance difference, or even a case capacity difference worth noting, between the Imperial and RUM lines. I'd bet money the R&D guys at Rem would've just rebated the Jeffery case, but they already fight the reputation of just putting their name on obsdure cartridges, so they made up a new case that was a whole 1/500th" bigger and made it "proprietary."