Vipper already said why 250 is NOT a good replacement, why opel is perfect -FOR OKW- and why OKW can't have any stock one.
So unless you want OKW infantry to be neutered, you're not getting one.
I don't agree with Vipper. The OKW Opel is terrible. Its aura has such a short range that you lose the halftrack if you try to use it so all it can do is skulk behind shotblockers. It plays like a 30 FU Ambulance without healing.
Replacing it with a 250 would be undeniably powerful, but at CP3 its counters are available and OKW already has the Flak HT/Luchs available. Furthermore, it'd be in the off-meta Feuersturm doctrine: unless it makes Feuersturm stronger than Overwatch/Special Operations it wouldn't be a net buff to the faction.
And if OKW really can't have a decent halftrack, replace that ability with something worthwhile. The Opel is a dead slot in an off-meta doctrine.