As an easy option, Rival total quest count should be introduced to 36.
Then two choices,
::::::First choice:::::::
each 9th quest in the rival produces 50 fragments of past year gold league building will be fair enough.
total of 200 fragments and one time achieving the building.
::::::Second choice:::::::
36th final quest task completion reward is past year gold league building.
But having it in daily special is a better choice for a one year old building. Keeping it in a event silver league is a compromising one, but each event (bronze,silver,gold) league rewards should present new rewards is the standard followed these many days. Epic and other selection kit kept in the league also newly introduced in every event.
As
@nice2haveu pointed out, daily special they should put restrictions for that particular reward to gain only once. Issue they complained is gaining that reward multiple times, what's the problem for them. One year some players struggled souch for that building to obtaining it, next year they can try to obtain without much trouble. What was the issue here.