MArcomage

Not sure what the general sentiment is (or how many people play this type of deck), but I believe the Illusion keyword is very underpowered. Here is why:

1. Sure, you get a rare, but it's essentially random since it's from your opponent. Because your deck is already gem-intensive, it's hard to utilize the Legend keyword to benefit from the rare. You could try to combo into Veteran militia, but this is already a 3-step process to get a rare from your deck (same for the bonus damage on Vulture). You can accomplish the same thing using Scepter of summoning much cheaper and faster.
2. The keyword overrides card effects. For example, you can't Doppleganger the matching card if your last card was a non-common Illusion (which is the point of the card). This same effect happens for Mirror strike, Mask of the phantasm, and probably others I'm not thinking of right now. This effectively limits your playing options.
3. The cards are insanely gem-intensive. So to use them, you have to fill your deck with cards that raise magic or provide gems. This doesn't leave a lot of space left for any utility cards, making you dependent on your opponent's rares (which you can't control).

I suggest we add an additional effect when the keyword is triggered:
1. Gain 1/4 of the casting cost of the summoned rare, or gem production x3, or refund the gems cost of the played card.
2. Have the rare replace the lowest rarity card in hand (selected randomly).
3. Reduce enemy stock by 1/4 of the casting cost of the summoned card.
4. Raise wall and tower by some amount (10?).

Sounds reasonable. I have an Illusion deck and it is indeed very gem intensive as you say. Illusion is a very fun keyword to play and does not need any fundamental changes as such, but could use a bit of a buff.
I think option 2 is the best one thus far; it resolves the issue of Doppelgänger etc. not actually having an effect beyond the keyword itself.
Comparing Illusion to the other similar keyword (Nature, Restoration, Destruction(, Aqua)) does leave little doubt as to which is the weakest of the lot.

Also, it's not like Illusion decks are overplayed.

Proposed option 3 seems like too much to me. I prefer the options that benefit the Illusion player, if anything at all.

Something to consider is that you don't need an "Illusion deck" to benefit from the keyword. I have a deck or two where I took an Uncommon Illusion card because it worked well with the rest of the deck and then grabbed a Common one as well to make the trigger possible. (Also, the occasional event of summoning Mirror elemental via the Unliving keyword which sets up the trigger by its own effect.) When used in cases when the non-Common Illusion was going to be played anyway, playing a throwaway Common for the trigger is pretty nice as it is now.

Perhaps what is needed instead, if we want to make the idea of an Illusion deck more viable, is to introduce more Illusion support cards that don't have the keyword themselves. "If #Illusion in hand > 3..." or "If there is a Rare in hand not from your deck..." and the like.

I'm with DPsycho on this one, some 1 or 2 support cards would be enough.

Although, gaining some resource of summoned enemy rare also is nice, but should be either capped,or be minimal like 1/6-1/8 or both.

With recent addition of cards like ancient power and ambassador, Illusion decks are not weak. Simply gem hungry.

Saw that. Thoroughly approve. Too late for me, but shall return to this later.

It would be nice to add some common illusion keyword cards which cost recruits and bricks, it would make the trigger more reliable. Now we have only 1 uncommon which doesn't cost gems - confusion (though has a huge drawback).

One good old concept for Illusion is this

In general I think Illusion keyword is fine as it is. It's just that Illusion lacks some good uncommon or rare attacking or defending cards.

I like the second option. it also get rid of possible keyword conflicts like Illusion-Nature or Illusion-Dragon

2nd option can make illusion spawn even 2 rares at a time. Sounds scary.