All your crazy theories concerning this show.

Leo is going to eventually fall in love with Kaoru causing friction between him and Kouga
Going to go ahead and say it: Leo has a crush on Kaoru. From the moment they met, Leo has had a way of looking at and interacting with Kaoru that appears rather more OTP-ish than being simple aquaintances. They spend time with each other outside of Makai business, she happens to remind him of a loved one from his past, and they even have their own "romantic" theme music whenever they are having a moment together. Add in the brush scene from Makai Senki episode 7 or the hand-holding scene from Makai Senki episode 10 (along with 90% of that ep), and it's a rather blatant Ship Tease.

Its probably going to end up Arthurian love triangle, with Leo being torn about his loyalty and respect towards Kouga (and possibly some justifiable pants-wetting fear considering that his rival can probably kill him twelve different ways from Sunday) and his growing feelings for Kaoru. It will either turn into a case of Love Makes You Evil (thwarted in his love for Kaoru, he'll end up allying himself with the Big Bad), I Want My Beloved to Be Happy (he will gallantly step aside and do everything he can to protect Kouga for her sake, even if it means a Heroic Sacrifice) or if Kouga does end up kicking it at the end due to the Seal of Destruction, an extreme, tragic case of Last Girl Wins.

Leo is the man in the red mask.
We know that the man in the red mask is a renegade makai priest who also happens to be a skilled swordsman, and in episode 10, we see that Leo is actually pretty handy with one. And this show has not exactly been shy about a character's seeming friend turning out to be the Big Bad.
  • This was a deliberate red herring. The man in the red mask is actually Leo's twin brother.

Leo is not the man in the Red Mask but he is close to him in some way
Leo and Red Mask do seem to have a lot in common- they are both Master Swordsman, they both want to be able to protect humans on the same level as the Makai Knights. But rather than Leo being the obvious choice for Red Mask, he is instead someone who is/was close to him. Possibly a former pupil of the Red Mask, which would explain why their principles are so similar, maybe even a former friend or relative whom he grew up and they both taught the same lore that Makai Priests were the original warriors against Horrors. But where Leo grew up wanting to support the Makai Knights, his friend/relative/mentor never outgrew the resentment he felt towards them.
  • If the spoilers are to be believed, he's probably Leo's brother.
  • Confirmed.

Makai no Hana will not be the story of how the GARO armour lost it's shine
Just so they can keep us waiting.

Kaoru will appear in Makai no Hana, but Kouga won't
His actor has apparently said he's done with GARO. This won't mean he's dead, but if he is it'll be pretty obvious he didn't die of old age.
  • Jossed in terms of any clear image of either of them.

The red-eyed GARO seen in Ep. 7 will become a Stealth Mentor to Raiga, and will be revealed to be his grandfather Taiga
. If only because it has been established since the 1st season that his original GARO armor had red eyes, and his spirit seems to be still roaming if Soukoku no Maryu was any indication. But then again most dead fathers (like Kengi in Tougen no Fue) seem to do so.

Rei will appear in an episode
Either he's still around and helped train Raiga, or he's in the same place as Kouga and Kaoru and will return when they do
  • Confirmed. He appear in #17 and was Raiga's teacher during his childhood.

The mystery of Kaoru and Kouga's location will not be solved this season
Instead it will be the driving plot of the next live-action season
  • The mystery of what took them is explained, but we don't know where they are, so the next season can do that with Raiga being the star again
    • In the Makai Retsuden finale, Kouga says he's lost between dimensions and still has something he must do. So sorta-jossed?

If Jabi is still around, she will call on Raiga at some point to continue the Bar Chess game
Kouga having called on her to continue it previously

The three Garo we see briefly in Makai no Hana's finale, from other eras.
Are Taiga, Kouga and Ryuga (assuming Eiris' branches can go forward in time as well as backwards. Otherwise I guess it's Leon Lewis for the third)
  • Could be Raikou.

The Saezima bloodline is descended from Raikou.
  • The live seasons are pretty Japan-centric, and Guren no Tsuki is the only animated season stated to be in Japan.
    • Jossed. A statement from the showrunners confirmed the anime seasons are in their own continuity.