Just an idea that was clanking around in my head that I had to get rid of.

It's like a Krasis Incubation but your opponent chooses which one to pacify. And while it only puts 1 counter on per turn, it is "free". At the same time, if your opponent has any other profitable blocks they'll kill the thing that the grove blocks.

Although, since the counters ARE damage, that means that if you assign 3 damage to a 3/3, it'll survive because it'll be a 4/4 by the time state-based actions are checked. That doesn't feel right. As eloquent as the current wording is (if it works in the rules), I really feel like this should use the stack to avoid that, just for grokability.

So, your opponent will always block your strongest creature because as soon as one gets to 6 power, they can lose the thing. Your strongest creature gets pacified, it gets a counter on it each turn until it's a 6/6 when it becomes a 7/7 and stops being pacified.

This card would have to find a home in some sort of "high power matters" deck with cards like Colossal Majesty so that the slow grind to get to 6-7 power matters more than the damage you're missing out on. It would of course have a spot in any +1/+1 counter matters decks. Most interestingly though would this find a spot in any deck capable in removing +1/+1 counters. Then you can swing for 5. If they block, you get a counter and you remove it for value. If they don't, 5 damage.

­Vigor also turns damage into +1/+1 counters, so that part works, although I guess it turns all damage into counters. I still think people's intuitive play pattern would be to account for the extra counter when assigning damage

While we're evaluating the card, I'm just gonna mention that auras like Oakenform will both speed up the rate you get counters, as well as boost the toughness of the Grove.

Yeah, I wasn't sure whether not to make that a trigger. In the end, I shrugged once I realized the same questions must have come up with the Wither and Infect mechanics. But sometimes people 'grok' towards what benefits them the most, so maybe Wither isn't such a great precedent.

The problem with trying to mimic wither and infect (and also lifelink and deathtouch which many forget) is that those keywords heavily rely on being hard-coded into the rules to work. Any reminder text that says "deals damage in the form of" or "damage dealt by this causes something" etc. is actually just that: reminder text. Such an ability is heavily supported by rules concerning the keyword and the text alone wouln't fly as rules text. If you want to make such an effect work without keyword, you should go the Vigor-way and type out a replacement effect to create an unambiguous well-defined ability.

I'd consider it an even worse waste to use Oakenform on a creature I gifted my opponent for still delayed rewards - if I get any and my opponent doesn't just have a sacrifice outlet.

The least I'd want is damage dealt in the form of Giving Grove's owner drawing that many cards for the effort put into it.