You could call it the Module-D.
If it's going in the EuroCrack direction, then it should maybe have a form factor similar to the Roland System-1m - a module that could fit in Euro, sit on the desktop, or be rack mounted but with an optional keyboard dock so that nobody feels forced to source a Euro enclosure and open up that can of worms when all they want is a Model D clone at the lowest possible price. Those that already have a Euro enclosure could save a few bucks by not buying the keyboard dock (even though it would take up a whole row of a Euro enclosure which is probably more expensive than a keyboard dock from Behringer would be), and those who don't need another keyboard could just slot it into their rack or sit it on the desktop. However I suspect that even users who already have a Euro setup would rather just rack the Module-D instead of using up a whole row of Euro enclosure anyway - that's what I'd probably do unless I was trying to build a very compact live rig or something. In terms of modular patch capability (if it even makes sense to go down that route), a simple row of the most useful patch points across the top or bottom of the unit should suffice, similar to what Tom Oberheim has done with his SEM-Pro reissue modules. If you add preset storage and recall, and MIDI CC control over parameters, you'll be offering features that no Model-D or clone can offer.
The keyboard dock could be very basic - it doesn't need to be a full EuroRack rails and power supply setup, just a keyboard+wheels controller that derives its power and MIDI i/o via a single ribbon cable to the Module-D, similar to the Roland Boutique keyboard dock, meaning it would only work with the Module-D. I think it would be a mistake to make it extremely tiny like the Boutique series, but a three-space 19" module would have enough room for fairly chunky knobs with decent ergonomic spacing, and would allow for rack mounting, Euro mounting, or a decent sized keyboard dock with full-size keys and wheels.
There are already a few units that go in this direction, like the excellent Waldorf KB-37 and the AJH MiniMod series of modules which let you build a fully modular Model D clone - but these items are not anywhere near the low price point that Behringer could achieve. The Waldorf enclosure lists for around $1,000 and to duplicate a Model D with AJH modules lists for around $2,100 - so you're already near the price of a real Moog Model-D reissue, although of course in this case you'd have full patch-ability.
I think the price point would need to be $999 or less for the combination of a patchable, MIDI CC controllable, preset-store-able Module-D and a Keyboard-D - but if the price can be driven down towards $499 by eliminating patch points, store/recall, MIDI CC control, and dock-ability then you'll still move truckloads of them. This would put it in the same category as the Korg MS-20 and Arp Odyssey reissues, but at an even lower price point. In my opinion, a bare-bones, fully-manual Model D clone with keyboard at $499 is a no-brainer purchase and would have broad appeal, but higher-end users might not see the attraction - however a CC-controllable, patchable, dockable Module-D + Keyboard-D combo, with store and recall at $999 would tempt even the most jaded of users.
If it sounded good, of course....
With all this said, I do own a Moog Model-D reissue, and have owned four originals over the last 30 years (along with countless original Arp, Korg, Oberheim, and Sequential Circuits synths) - and the reissue is my favorite Model-D that I've ever owned. The sound and build quality are beyond reproach, beyond compare, and worth every penny - but I admit that its price point places it out of reach of a wide swath of potential customers.