Going minimal here -
off colour deck skeletons analysis:
Entropy (Nymphcord):
4x Discord (Required).
5-6 Pill/Pends
3-4 Antimatter
0-2 BE
12 card minimum investment for a very potentially powerful deck. I'd say it's very worth it.
Death (Speed Poison)
3x Arsenic
4x Bonewall
6x Chrysaora
0-6x Poison
~13 card minimum investment for the best core. The issue with this is that it's not so powerful and the other cards are hard to splash, excluding arsenic. Going the full poison route only increases this investment. I'm hesitant to run this deck, since we don't have the hard CC to make bonewall actually as strong as it can be and it can take a lot of card slots.
Gravity (Momentum)
5-6x Momentum
0-4x BH
Yeah, that's about it. We can go cute by snagging black holes etc. but with only 5 cards to secure ourselves a Dimbreaker deck I don't see why we shouldn't invest in the core. Momentum is also a fine splash in novabows so having 5-6 should never go wrong here.
Earth (Trident)
A quick look at this shows me that any route we go here is really card heavy. It's going to be a 50/50 split either way and as far as denial decks go Nymphcord is just stronger. Our stall option isn't much better, with the Pill/Pend requirement taking up an insane amount of card slots. I don't like it, it's not even one of our strongest decks.
(Spirit/Adrenaline)
I don't like this. The investment is for subpar cards and we can easily get better cards elsewhere. Our minimalistic approach maybe does this:
4-6 Forest Spirit
4-6 Pill/Pends
I do like at least 2 Spirits for the waterbow but the full duo doesn't seem great to me.
Fire (SteamFlaggers)
This is awesome.
4x Deflagration
4-5x Steam Machine
2-5x Rage Potion
0-2x Fire Buckler
0-2x Rain of Fire
Throwing like 12 cards into this gives us a great domin. We can easily take two. I like this for one of our core options.
Light (Sancstall)
6x Sanctuary
2-4x Miracle
6x Icebolt
8 cards is a pretty light investment for a core potential light stall, especially compared to our earth duos. We can go deeper into it by taking pill/pends and NTing nymphs but we don't need to. The Icebolts are meh but they go well in this deck and are borderline in a lot of our other decks. It'll at least be always on colour and access to hard cc.
(Misc. Air duos)
So much for so little.
3-4x Wings
2-3x Fog Shield
0-2x Longbow
2-6x Shockwave
0-3x Owl Eyes
We can literally make an air duo with 3 wings. And we have no reason not to. Awesome, and I would go fairly deep here.
Time (Reverse Time)
4-6x Reverse Time
0-3x Eternity
Only reason to duo with time is to pick up RT. The synergy is not strong, but 4 is enough to maybe make a splash duo and can be thrown into our novabows. It's a very good metagame card so I can see us taking it. 4 cards is very little investment.
Darkness (Nightmare)
4x Nightmare
2-4x Steal
0-2x Stiletto
This is another really cheap and potentially strong deck. I'd take at least one, and we can afford two.
Aether (All the things)
I'm biased but I'd like to go deep into aether. They're just such a strong element and synergizes really well with water actually. This duo gives us so many power cards and versatility I don't see why we shouldn't take this as our main secondary.
Nymph's Tear Rush
12x Pend (Are we allowed this many?)
0-3x Quint
Pends will work in any aether duo so there's no reason not to go deep here. The quints would mainly be for this deck but they are really good on squids.
Lightning PUgon
4-6x Parallel Universe
4-6x Lightning
Parallel universe is the main investment here but PUgons is worth it IMO. It breaks a lot of stalls and our dragons don't die to opposing Lightnings. 4 is a good number for a lighter investment, but lightning is a deck on its own and can be used anywhere so there's no reason not to take 6.
Dim stalls
4-6x Dimensional shield
My favourite use of dim shield is to throw in 2-3 into a deck. It's just light enough of a card investment that running into opposing PC won't destroy you, but enough to win games against the decks it's relevant against.
Fractal decks
1-4x Fractal
I wouldn't mind having a couple of random fractals in the vault even without a dedicated fractal deck. It helps a lot of slower decks maintain advantage late game and breaks stalls wide open.
Other random utility cards I like to have:
0-3x Holy Flash
This card just rips death and darkness to shreds, which is plenty enough reason to run it. It's kind of a dick move, but I like having 3 in the vault.
That's my two cents.