Soo, possible plot parts for season 3:
-) Tension between humans and Youkai (including possible war scenario and so on)
-) Witches (it's not like they can just forget all the witch hunts)
-) Human experimentation to create a human with superhuman abilities (Tsukune pretty much proved that it is POSSIBLE to pull that off, although with the help of the "holy lock") and all the conspiracies that come with that
-) Somebody (finally) comes up with the idea to get Tsukune to do bad stuff by threatening his family (since it is pretty much impossible to defeat Tsukune without leveling a city or two now, the easiest way to get him to obedience is to abduct/threaten his family)
-) Everybody at youkai academy knows that Tsukune used to be a human (Because what's scarier to youkai than a human in Youkai academy- a human who is stronger than anyone else at Youkai academy)
They will (at least a number of them) be scared of Tsukune because if he tried and Moka wasn't there at the moment to stop him, he could annihilate Youkai academy all by himself (Mikogami usually seals his abilities when at work as headmaster so all Tsukune would have to do is get rid of him before he transforms)
-) What will happen if Tsukune's (human) friends/family find out he became a vampire, a shinso on top of that (The thought of him being capable of reducing the number of inhabitants of ANY building to zero in less than five seconds or casually juggle some cars sure is scary)
-) Maybe a new human will enter Youkai academy, a girl perhaps
Well, now that Mikogami admitted that he made the holy lock so Tsukune would become a shinso/vampire we can pretty much safely assume that he was the one who got him into youkai academy in the first place (and Hokouto too, probably)
In season 1 it was said that his dad picked up the application form that a priest accidentally dropped and that the priest gave him a creepy feeling. That is quite an accurate description of Mikogami when he is not in his Youkai form
Also, Youkai academy isn't something you could just sneak into, just like human schools it probably has some family registers and whatever in order to be able to contact the parents in case of emergency. Also, despite them not really being allowed to show their true forms to anyone, they would still have to disclose their true form (if they get hurt, that knowledge would be needed for medical treatment)
In other words, Mikogami planned all this, the plan was most likely:
1) Get human boy into my school
2) See if he can survive and hope that he befriends someone there
3) Set up circumstances which would lead to him being injected with Youaki blood (if it doesn't happen without his interference)
3) Wait for him to start losing control
4) Give him the holy lock and say it is to "seal the blood" and shut up about the "evolve into Youaki" part
5) As soon as he uses his abilities regardless of the lock, give him a choice to leave the school for his own safety
6) If candidate deemed suitable, train him so he can be able to fight Alucard/his forces later on
7) Bring him to the final battle
8) Hope he doesn't go full rage and kill you with shinso powers
Hokouto didn't make it beyond 5), he was most likely not deemed "suitable"
Moka's mom disappeared on Moka's 10th birthday, Moka is 17 and has birthday on 11th May (lol my Birthday is on 11th May too). If Mikogami sent one human every second year to Youkai academy, then there would have one or two more humans that have been admittet to Youkai academy, IF Mikogami started with that one year after Akasha's disappearance. They have not been mentioned yet, but they might make an appearance if season 3 came out