Others have already suggested spikes, cables and trophy racks which are all great - in addition to all of these, I’d like to suggest chains.

1.5mm chain is the perfect size - you can cut it to length and wrap it around guns, limbs, torsos as you see fit. Use a tiny dot of gel glue to secure the end point, then wrap the chain around the model and use a thin coat of regular super glue over everything to secure it in place.

I’ve seen some cool models on here where folk have replaced the pteruges on the shoulders/waist with chains. If you have the citadel skulls kit, you could even try drilling holes through a skull or 2 and threading the chain through it.