The most logical option to figure this out is bringing the off-hands to the same level as they are on RS. Therefore, nerfing them to the ground. An alternative way would be buffing (doubling) the stats of two-handed weapons as well as shields. If you don't want to affect the way PvM has been for a while, you could do this. The way things are currently also makes PvP impossible.
The way dual-wielding works is absolute non-sense. Having a Rune scimitar as an off-hand is a better option than having a shield of ANY KIND? Currently, dual-wielding is the only viable option for combat. Two-handed weapons, as well as shields are absolute carbage. I just heard about the bonuses that off-hand weapons grant today. To me this doesn't make any sense. To be honest I don't mind how you guys figure this out, but it has to be figured out.