Inconsequenti-AL
Breaks Games
Personally, I'd go for a lich as the BBEG. They're great - make them high enough level to give the party a run for their money, without the frustration of getting repeated beatings from an 'uber' NPC... Even if they get lucky and kill it off, then it'll be back for more. Finding his phalectry is going to be the only sure fire way to put him down - and that is as easy as you want to make it
- 9th circle of hell? Archmages tower in downtown Menzoboranzan? Etc.
Even better if he uses illusions to appear human... Someone will get a nasty suprise.
Anyway - few thoughts:
What does the BBEG want?
Is his/her first way of getting it going to be war?
Why can't they simply teleport/kill/steal their aim? - perhaps they physically can't go near it?
Some ideas:
The capital of Noble-Land has a demon prince buried under it. Wants to release him for whatever reason... Unfortunately he's imprisoned in a cathedral BBEG can't enter or affect with spells - needs to get that knocked down...
BBEG must be crowned the legitimate king of the noble kingdom as part of a prophecy. The court of nobles needs to elect him/her. If the current crop won't do it, then they need to be changed?
A little more shade of grey - perhaps BBEG was the legitimate heir to noble kingdom, but was cursed by an evil brother - the current kings great-grandfather? Maybe he needs to destroy the bloodline before he die? Trouble is, nothing he personally does can injure them. He needs an army...
A dragon could make for a great 2nd villan or 3rd force. Perhaps it's the ruler of the neighbouring lizardman tribe?

Even better if he uses illusions to appear human... Someone will get a nasty suprise.

Anyway - few thoughts:
What does the BBEG want?
Is his/her first way of getting it going to be war?
Why can't they simply teleport/kill/steal their aim? - perhaps they physically can't go near it?
Some ideas:
The capital of Noble-Land has a demon prince buried under it. Wants to release him for whatever reason... Unfortunately he's imprisoned in a cathedral BBEG can't enter or affect with spells - needs to get that knocked down...
BBEG must be crowned the legitimate king of the noble kingdom as part of a prophecy. The court of nobles needs to elect him/her. If the current crop won't do it, then they need to be changed?
A little more shade of grey - perhaps BBEG was the legitimate heir to noble kingdom, but was cursed by an evil brother - the current kings great-grandfather? Maybe he needs to destroy the bloodline before he die? Trouble is, nothing he personally does can injure them. He needs an army...
A dragon could make for a great 2nd villan or 3rd force. Perhaps it's the ruler of the neighbouring lizardman tribe?