... but why? I can respect the mental exercise, but basically cumulative upkeep is a rather complex way to make players do something they don't really want to do (paying mana costs before you draw your card). I don't think it's usable at common in NWO. It's also a drawback mechanic and likely restricts you from using any other counters (such +1/+1) in your set. Somewhat similar to fading/vanishing but maybe even worse.
IMO a more productive project would be finding a simplified or "better" cumulative upkeep mechanic that still does the things you want it to do. This really depends on you what you want to get out of cumulative upkeep (mechanically and flavorfully).
Like maybe, from the top of my head
Creature - Beast
Age (At the beginning of your upkeep, put a level counter on this.)
Level 2+: 3/3
(maybe something like "Remove two level counters from this: Regenerate this.")
At first I was using age counters and "Age 3+", but then I realized one might use just levels instead.
Add in some Paradox Haze effects and it's a pretty cool stuff I guess.
Also, your description says "cumlative upkeep" and the markdown syntax doesn't seem to be parsed there (so it's surrounded by two asterisks from each side).
You seem to think that making this set and trying to find alternatives are exclusives. This set is a "mental exercise" and will force me to go down paths I would have otherwise not considered - the flavor of the set is one such thing.
The plan is to finish this project and then use the lessons learnt from it going forward e. g. I can already see myself revisiting the flavor of the doomed without being encumbered by a mechanical prerequisite ("Valhar Director's Cut" maybe using a stand-in mechanic like age).
Cumulative upkeep is actually not all that much restricted by NWO. The mechanic uses a lot of text but NWO allows provisions for this if all of that text is reminder text (the usual rule here is that a keyword that alone has multiple lines of reminder text will be allowed one or two more lines without necessitating a red flag).
The other possible problem is that cumulative upkeep is problematic in large numbers. The current skeleton is putting those numbers at 7 common cards with cumulative upkeep of 86 common cards. I'm willing to use half of my red flags on those cards if necessary but I'm also happy shifting some of them up to uncommon.
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