Maybe a his and hers, a bowie hunter for her and a bird and trout for him (or otherway round, whatever)
If you want to go art, then by all means, but if you want to get them users, then I'd go for some good kitchen/BBQ knives, something that will have a connection while its used.
With 500$ in the budget, I'd be talking to the custom makers in the crew here and see who can come up with some ideas. If its a three or four piece set, it can also do duty during the reception for the cake, and maybe more depending on the dinner style.
As for engraving, I'd get a nice display box done, and leave the knives more plain, so that as they become heirlooms, they don't loose anything, but also have a block or rack done up so that they are usable, even if they are just "special occasion" knives for big family gatherings.
If you want to get a bad-ass custom for them, then I'd leave them as stag/staggette gifts, since those are more individual, but for a family, food is important.
