Having Old Guard get a bonus for Cav would be a good idea if the same trait sysem is used in HOI2.
You can always promote old guard to FM if they are MjGen skill 3, LtGen skill 2, or General skill 1. Then have them command one division in some remote area so that you can avoid the -15 no HQ penalty when there is no FM in or adjacent to the province.
If they also have some decent skill and another good trait like defense or trickster, they can have some other uses if no one else is available or you don't want to bother to track now a different defensive Lt Gen.