So, this leaves UR and URG as options and I prefer 3 color decks in general so URG makes the most sense. Magus Lucea Kane, Rashmi and Ragavan, Omnath, Locus of the Roil, The Swarmlord, and Xyris, the Writhing Storm are the top contenders for these colors. All have their own pros and cons both in terms of where they lead the deck and in terms of power level. I don't want anything that screams "answer this immediately" (so no Animar, Soul of Elements) and I don't think any of these really fall into that. Maybe Xyris as the best way to break the symmetry is to prevent draws but even then I think those effects are few and far between.
In the end, I felt that Rashmi was too open-ended and I don't really think the idea of Elemental tribal appeals to me (though I might revert on this eventually). I like that Magus and The Swarmlord are focused but still leave a lot of options on the table. Though, The Swarmlord is a bit more open and it gives me a reason to do more with things like Keyword counters which I think can be fun. But, of course, being 6 mana is detrimental as it is one that might be tough to recast last on even with ramping.
I think I will have Magus at the helm due to mana cost and due to letting me do more with big X spells like Genesis Wave and whatever else I find The Swarmlord probably makes sense in the deck since a lot of X creatures deal with counters but neither The Swarmlord nor Magus Lucea Kane exist online which makes testing this deck more difficult than it should be. So, I will just exclude The Swarmlord and have to test out the bulk of the deck without the "right" commander. Losing the Commander is pretty detrimental as there are some cards (such as the two Candelabras and Helix Pinnacle) that are here primarily for their synergy with her.
Online I am going to use Surrak Dragonclaw simply because it is a creature heavy deck and with all the counter synergies, having Trample is a pretty good option to get in damage. Plus, there wasn't a good option that closely replicate what Magus does. Losing the Swarmlord isn't as big a deal as it really just drew me cards but there are plenty of other options for card draw so that is why I am just fine cutting it entirely.
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Here is the list I am starting with and I will be playtesting quite a bit and tweaking things a bunch as I figure different things out.
As noted, there are a lot of X spells and a few X activations (though not many). Unbound Flourishing adds another similar effect which does make testing online a bit easier when I get to that effect. I will admit that Helix Pinnacle is here because it makes sense but it might end up getting cut as I am not the biggest fan of "I win" cards. Plus, it also paints a target on my back which I am not sure I want to do.
Beyond that, the idea is basically to flood the board with creatures, add counters to them when I can, and swing. It is a pretty basic idea but it does so in a way that is meant to complement the commander.