MArcomage

Is there a possibility of having some extra cards, which cannot be added to deck, but can be summoned?

Summoned either by keyword summon, from the whole pool.(summons uncommon holy, summons rare beast)

Or maybe by playing a ritual of sorts:
If mage tokens are 53 and lord of time and fairy are present summon Bunny!

It has been proposed before and I'd personally be open to the idea. It shouldn't be a problem technically as this sort of card already exists: "Empty". You see it every game :)

neat

It could be one universal "ritual" card, with link in description. And that link would lead to single webpage with all the formulae. That way there will be more functionality, and just one extra card.

As per extra summon-able cards - they could have combination of usual keywords, and a common keyword, something like "forbidden" or "grey" and some 2-3 cards to affect that keyword.

And the cards should be not very powerful. Like 7 on a scale of 10.