I have to express a few sticking points here.
From the first, this passing fancy of "combos with
Pyromancer's Goggles" just seems so out of place. While I'm glad Goggles has caught on here on Riptide, it isn't exactly something I think needs pushing across colours, and certainly not off the back of a 7-mana, mana-doubling, cares-about-creatures
enchantment. After all, a double
Fireball at 7 mana is already itself 12 damage, which should be damn near game-ending, and if you're going
ramp-and-red-spells-matter line, you can probably get by better and more consistently off of other ramping tools. I'm interested in ZR myself, but this looks like a trip in the wrong direction to me.
Your proposed blink shenanigans with
Soul of the Harvest also necessarily ask the caster to be in a
or
shell, with at least 1 other creature on the board, to draw a card. This is asking a lot I think and feels very win-more, as well as already being done quite well and quite boringly by old standbys like
Sea Gate Oracle; similar to the "don't make
worse so that
can do it equally" point made early on in this thread, I think "don't try to promote a boring
theme as a boring
theme" should be considered here, especially since 99% of the time, the effects will just be tucked into other decks.
Moving more specifically into bigger, more theoretical contentions...
While you're fair to criticize enchantments as being typically noninteractive in most cubes, I think that's probably not the best way to be approaching expensive control cards. After we get above 5 mana, things really need to have an impact in most formats, and especially aggro-friendly Riptide lists. Sure, ZR is potentially impossible for your opponent to remove, but for a 7-mana enchantment that almost certainly won't start paying off until your next turn, I'd say if you can untap with it on the board, you've damn-well earned the right to get some use out of it - some use, which I might add, won't always necessarily be game-ending.
Further complicating the maximized-interactivity-is-fun theory you're using against ZR is that it appears to be, from my perspective, self-regulating. I mean, you say above that the bodies on these creatures make them a potential "win-con" for being able to swing
and generate value while also being balanced by being removable; is ZR not balanced equally by being hard-to-remove (but not impossible, mind), capable of generating value, but
unable to swing at all? If anything, this seems like precisely where you want to be in a thread about "Starting [one's] Engines", right?
I mean hey, your format is your format, do what you want with it, but I think you have to suspend a certain expectation of interactivity from time to time. I personally think it's really fun, flavourful, and cool that enchantments and artifacts are harder-to-remove, similar to how I think it's really fun, flavourful, and cool that counterspells are almost always blue and burn spells are almost always red. It's a dimension of the game that
can be busted to high heavens if you allow it to be, but will often just be a cool thing your drafters come to expect and, under repeated exposure to such cards, will begin to prioritize removal for. FYI
Pyromancer's Goggles is in the same hard-to-remove camp and is wildly more playable and more likely to end the game in a hurry than ZR is, so I guess I'm just skeptical of this "noninteractive" argument, since it seems to be missing some serious culprits in your own list if this is a concern, culprits which, while I agree are hella cool and hella fun, are similarly powerful thanks in large part to being harder to remove.
With regards to creature specifically (and more specifically, the ones you've suggested), without an ETB trigger, 6+ drops are typically too risky in most Riptide formats, and in all these proposed cases, it looks like the potential rewards don't really offset the risks. I personally can't imagine risking eating the tempo loss in any format of dropping a
Primordial Sage, because it could easily die before I can generate
any value off of it, and being a
6-drop that belongs in decks that cast creatures (thus rendering it less friendly to cheaty bullshit), that's really not great. And of course,
Regal Force suffers for being a card that ramp will rarely play and that reanimator has little interest in as well, but I'm at least more open-minded about that, because I can theoretically see some really low-power format that wants for it. But these options feel very lackluster to me, and, looking at your own cube list, all fail to compete against your other 6+ creatures (
Moltensteel Dragon excluded). If you want creatures as low powered as
Primordial Sage,
Soul of the Harvest, and
Regal Force to make an impact on your list, you'll need to tinker down removal power level a fair bit, as well as stripping away virtually all of your current, very bomby 6+ drops. But honestly given the kind of strong environment you've got, I can't imagine many decks are going to have the time to drop ZR; I think if someone builds a deck capable of making that time, they should be rewarded for it, and your format looks pretty ideal for such a big bomb since it's really 1 of many, but again, I totally respect if your design philosophy holds you back from taking that leap, we've all got our hangups (mine's ugly art)