Well, if you cough up the extra 8 natures, you can always Transform them. When you do this (similar to Ritual of Rebirth and other life after death spells), you can then resummon them since the game doesn't think they are summoned anymore. The transformed TL still will get stuck with few slots, but at least they are mobile.
Nowhere near as useful in 99% of the situations as the Fire Kings or Air Queen, so I'd like them improved. Some thematic ideas to give to a variation of them:
a) Make them Entish, so they can walk
b) Free auto-attract Vinemen
c) Free auto-summon allies with Vine Ogres/animals/Crossbreed
d) Free Fairy trod ability
e) increase population/growth scale in occupying province
Free Fairy trod could be too much,but the forest only restriction, low leadership and only one available wouldn't be overwhelming ...probably
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