I know this has already been discussed to death, but really, I believe the most attractive realistic DLC which can be made rn is the habitat bird DLC. The recent addition of the swan rig makes things way easier. It's crazy the amount of habitat bird which can be covered by the swan, ratite, peafowl and flamingo/red crowned crane rigs alone.
Something like this :
- the great white pelican (based on swan rig)
- the mallard duck (based on swan rig again)
- the golden pheasant (based on the peafowl rig)
- the rhea (based on the ratite rig)
- the grey crowned crane (based on the red crowned crane/flamingo rig)
- the secretary bird (based on the ratite rig with a few new animations)
- the scarlet ibis (based on the crane/flamingo rig with a few new animations)
- lorikeet or hummingbirds in the walkthrough habitat (new rigs here)
It would have a clear headliner in the pelican, many very recognizable animals (duck, golden pheasant), a few community favourites with the GCC or the secretary bird... This is not my favourite lineup, but I think it's the one which would be the most popular.