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What exactly would these bases do? Just be a safe little farming area where no one will touch your trees?
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I think they quit because of boredom, they only can control one unit. If you added some kind of income system for the home tree, it would boost the amount of interest for players. At the end of each round they'd have the time to work on their economy with the home tree. For example, have roots under the home tree, use root model that automatically grows when you increase economy to make it look cool. Make so that only the home tree owner can see the roots with a little opacity. If allies are forced to see it, make everyone neutral to each other (expect enemies).
Here's also a suggestion on how the home tree would be planted; Have an un-droppable item taking inventory slot 5 or 6. Its tooltip would say "you require x amount of seeds, x amount of logs and x amount of water to construct the home tree". The item would trigger point target ability, if player has ingredients, it would start constructing home tree. At first home tree has little to none roots, once you upgraded it, you would see them grow, and you can start seeing +1, +1, +1 on top of the home. Then you can boost the number. After constructing the tree, the item would disappear, but the slot'd stay reserved. If the tree died, make the item reappear.
Now what you would get with the resource, would be a weapon which boosts your attack drastically, maybe add some effects to it too. Of course they would cost a lot, like 500, 1200, 3000, the weapon could be sold for nearly the same price so you can easily get the next weapon etc. You could also buy a single usable horn that warns the sentinels about threat, then it would trigger the sentinel forces to appear.
Other thing you'd get from the tree would be some kind of ability to connect the roots with other tree. Again other players could not see the roots. It would drastically increase the trees health, and make it so that it's harder to chop down. Also it would increase the economy.
I could go all day about ideas but I don't want to ruin my keyboard/your VMs... And these are only ideas, you're free to go your own way.