I'll be honest the first thing I looked up was Dendro characters and was a little heartbroken there were no PCs yet and few mechanics. After I've been playing for a bit I realize there's a lot for other elements but I still think about it sometimes. I even thought of trying to build a Dendro OC at some point even if it wouldn't really amount to much. But I figured I could at least spill some of my brain sludge here.
A bit of D&D inspiration here I think it could have the potential to create slow terrain. Not thorns for damage but vines, it could even stack leading to being stuck in place. Almost like bubbling but a slower build, I think Dendro could be a debuff element very easily. Dendro characters could have vines or thorns for their elemental skill/burst for variety so it's not just thorn blasts everywhere and maybe a tree for a tank or flower for healer or magic user.
I think the effect could even be called Tangled or Ensnared and have natural patches of viney or overgrown terrain similar to water being naturally occurring.
Reaction ideas:
Pyro + Dendro - Burning (fire damage over time)
Hydro + Dendro - Grow (could affect vines for increased stacking of slow movement or affect thorns for Dendro damage over time.)
Cryo + Dendro - Shatter (AoE of Cryo damage similar to overload)
Electro + Dendro - Battery (x2 Electro Damage when Electro triggers the attack. Not as sure about this one but I wanted to make a potato battery reference somewhere.)
Geo crystalize and Anemo Swirl as always but--
Maybe Dendro + Anemo make a sort of poison? I joked about it being hay fever but it could still make sense like a cloud of spore effect Damage Over Time still the same range as swirl would normally take it but poisonous.