I think that either is fine, as long as theme is sound and the strategy is good and typically (not always) well known.
For example, look at
JCJ's Aftershock.
It has a good strategy (Discquake) combined with a side strategy of buffing Wardens with CP for control. It's also flexible, since it contains both nasty PC and CC and creatures are very CC resistant.
It's countered pretty well by a PA'd Gravity Shield with decent healing (perhaps a stack of PA'd SoG, or just straight up Stone Skin). While it is very powerful, it can still fall to counter decks, which I personally believe to be important in designing. A FG should at least be able to be countered reliably if known beforehand; otherwise, your FG is likely too strong and needs to be toned down a notch.
And that's just mechanics.
Thematically, it's not as concrete as others (like Dark Matter or Ferox, two of the FGs that I believe to have some of the best themes compared to their name and deck), but still sound. It's concept is almost based on the butterfly effect in real life, where one thing can lead to a mass chain of events that ultimately end in your downfall. It's theme is great (at least, IMO).
And concerning card usage:
Having new cards is always nice. After all, having different variations of speedpoison gets boring.
But obviously, including cards used by other FGs is perfectly fine AS LONG AS that card is either:
- A main card in your FG and a minor card in the other FG (think taking Cloak from Serket)
- A minor card in your FG and a main card in the other FG (think taking Forest Spectre from Elidnis)
A "main" card would be one that has 5/6 copies, is almost always seen in games against that FG, and is a key thematic and strategic card in their deck.
Puffer Fish is a prime example from Scorpio, it's Scorpio's trademark card.
A "minor" card would be one that has 1-5 copies, is sometimes/rarely seen in games against that FG, and is typically more of a strategic card for the deck.
As mentioned above, Cloak from Serket would be a minor card. It isn't particularly thematic (unless you take into account the lore behind Serket, but I'm not), and it is mainly there for the mechanical benefits in creature protection.