"Ultras" is an interesting concept, but it could only work if people actually knew what "ultras" are, and what they stand for, and reading this thread many or most don't.
Its my opinion that there is a much easier way to have a better atmosphere, and that is to create a section in the ground where the seating is unallocated, where people arrive, and take up the 'seat' they want, rather than sit where they are told on their ticket.
This is how the best atmospheres of the were created at all grounds before the Taylor report. It doesn't need "safe standing", though that would be a help in my opinion, and it can still be season ticket holders only.
Create an area of say 3-4000 where you buy a season ticket for the area, rather than the seat. The club would only sell the number of season tickets that the area will accommodate, and you can only gain access to the area if you have a ticket for it (would need an extra layer of turnstile I think), this would require some sort of segregation to prevent others trying to join, and causing over crowding.
The people who would buy for this area, would be told that its likely to be an area where standing is likely, and that they are not guaranteed the same seat every week, and would accept these terms from the start. As one of our polish friends says above, it needs people with the same ideas/mentality together in the same area, people that would sing, would want to sing.
There are issues with safety, and I'm sure the club would need some convincing, but these could be easily got over with a bit of common sense on both sides.
It might even generate an "ultra" type culture through time.