Off-hand elemental spellbook: An elemental spellbook that charges when you hit an enemy and when fully charged, it gives the elemental damage to your weapon - you could think of this as a rework of the runes, when it is equipped, the spellbook eats away the 1st 50 focus you get when hitting enemies and then trigger this, its like an automatic way of setting those runes in the form of an off-hand. Once fully charge you get focus as usual obviously.
Edit: Or an empty tome which you equip the desired rune to trigger automatically through method above
Duel battling shield: Take inspiration from the weapon shields from elder ring dlc, since I cant use a shield on my main hand, make it a weapon
Longer spears: Pykes and polearms etc.
Transforming weapons like charge blade in monster hunter: Weapons where you need to do a combo or parry or wathever to unlock its second form, alternatively weapons that eat away your focus similarly as explained above for the spell book and activate their second form when the right amount of focus is consumed
Give your interesting suggestions below (if any)