MTG Salvation

I've been brewing some ideas for a new Custom Green/Blue Commander. I want something like a Giant Kraken that has control over the landscape around it, bending mana to it's whim. I was thinking something with a Tap/Untap effect, letting you pin down your opponent's resources as well as untap your own, and trying to avoid as many "Go Infinite" combos as possible. I was looking at cards and trying to get an estimate of what the average cost would be for tapping or untapping a permanent. The distribution is rather wide, upon inspection, and so I was looking for some ideas on what everyone thinks is a good, reasonable cost for this kind of ability.

Proposed Idea:

Aglothos, the Sea's Wake 6GU
Legendary Creature - Kraken
GU: Tap or untap target permanent.
[Other abilities yet to be made]
8/8

Looking at the 50ish permanents that tap a target for a cost that doesn't involve themselves tapping. Taping a creature is about CC meaning you could reasonably get any permanent at 1CC. While untaping is trickier to find, though it could reasonably settle at 3C. If the effect did both as you have it, maybe 2UG?

goes infinite with Gilded Lotus, which many high-curve decks are going to want to run anyways. Astral Cornucopia as well, though that costs more.

Though untap effects are pretty much always going to go infinite, and costing 8 makes that somewhat more reasonable.

Gilded Lotus is one color at a time, though, as is Astral Cornucopia, so being two-color means it doesn't go infinite off those alone. You need some infinite filter to do it at two, and the "standard rate" tends to be about 3 mana anyways. So 1GU seems fine, bit of a danger when you get ability copies or discounts involved, but there's not many such effects that'll recur reliably, making it a low-reliability combo unless you really dive into it.

If the "theme" is landscape shifting and bending mana around, another ability could be a Scapeshift effect, alongside some form of color-hacking ability. Maybe use Islandhome and Forestwalk, then the hacking ability lets you mess with the creature's keywords to match the opponent you want to hit?