As discussed a lot before there's a lot of reasons to play on freedom over pvp on NFI - you can make money and get extra skillgain. As time goes more and more get the extra skillgain and have less reason to bother with Defiance.
See some of these topics for more info:
To make a flavourful Wurm-themed I suggest adding seasonal bonuses on Defiance only. These still won't make it more attractive than the freedom side, but at least add to balancing it.
Rotating between buffs every week or two (or attached to Wurm seasons somehow):
- Increased affinity chance (2x, I'm sure most ppl still won't get any skilling)
- Skillgain bonus (25%)
- Rarity bonus (2x)
- Increased mob SP drop chance (2x)
I just suggested some initial numbers based on the fact that its competing with easily buying SP and path of knowledge skillgain.