and why not a mineral druid?
he could link with stones....
for example building cromlech through the arena to encrease his power!
This is interesting. How about a Druid trained in Nature and level 1 Earth instead of Water. Abilities might trigger around walls, altars and/or temples.
Problem is that such Druid won't bring tools for current Druid to use.
Alternate druid could either use Vines in different way, for example casting spells similar to Warlord's Battle Orders from them like giving target creature rooted. This would put Vine Tree in even more dominant position for tree of choice. Other option is to give him strategy that would use other trees better, for example healing/gaining life whenever You or your spawnpoint casts plant spell. This would put Samara Tree+Seedling Pods as dominant strategy for alternate druid, because casting Seedling Pod would trigger your ability twice.
Nature- and Earth-trained mage could be desert/insect mage coming from southern continent.
If You want mage focused on cromlech, walls, altars and/or temples I wouldn't limit his earth training to only first level.
Guys. Guys.
We are ignoring one of the most basic druidic skills.
Shapeshifting.
I thought we're playing MW and not D&D. I'm not against shapeshifting mage, it's just that it wasn't only druids who were shapeshifting in celtic myths. Even St. Patrick could do it. It's common thing to do like witches turning people into frogs.