We already have 5-day Needles of the Tempest, Dragon Abbey, and Sanctuary in the ELR, MMM, and UUU but what about the rest?
Mini versions of most wonders could be made:
an ETC that makes your MM spells more powerful and/or pumps out a lot of seeds
A Maze that pumps out mana and/or reduces mana decay
Etc.
The key is that by offering 5-day expiring buildings they can be made powerful enough that players would actually want them without ruining the game balance or inflicting more power creep.
The enhancement to strategy is potentially fantastic.
For example, I'm at a stage where I've decided to add a couple of factories from zero to max, and that will cost a few hundred thousand seeds more than usual so some "throw-away" seed producers and/or some that reduce seed decay would be excellent.
Likewise, when working on a MH upgrade or some costly techs you could toss down some Mini Mazes for a little mana boost/decay reduction.
Adding in VS, and HF minis could actually make melee much more useful in fights too, breaking the "Range>Everything Else" in current combat.
(this would be harder to balance than the rest, but that can be handled)
Even the 3h troop production mechanic from wonders could be used. As long as it makes more troops than an instant at the cost of taking up space for 5 days and requiring interaction via 3h collects, I think it would be a desirable alternative prize.
Well, I think you can see where the idea is going, so do you think it worth pursuing, and fleshing out? I don't want to waste the time if it's not something that players would find enticing as a prize option.