Kinda loaded question as it ends up in another worst UB debate. Probably helps to post a list of generally accepted terrible UBs, which can be found in a poll thread somewhere. I do agree that UBs in general are not game changers with maybe one or two exceptions, and even those UBs are attached to marginal civ/leaders.
First, in response to KidR above, I think the Mint is fine as it is and a very good UB. I build forges almost everywhere regardless of base hammers, as the building enhances whipping in high food cities - cities that can later run Mint boosted hammers. Forges are not just for hammer cities. +25% seems a bit OP to me.
As for the Mall, I think it is actually an very good UB that is made basically irrelevant due to the timing in the game, as one may never even reach the Fridge in many VC types. However, it's great for late game rush buying and happiness in Space games or in late era starts. IMO it is a "flavor" UB for a civ that logically would not have a special UB in early eras. However, it is quite strong for its time. So for the Mall, I don't think the outputs need to be change. If anything, it could arrive a tech or two earlier, but then what building would it replace.
Research Institute - I'd rather have a small boost in space parts production. The extra scientists really don't add much late game.
Salon - Small boost to research or +2 happiness or health. Nix the artist
Feitoria - While keeping the +1 to water tiles, reduce the cost of the building by say 25%
Dun - Yeah, a wall UB is pretty crappy, but I like the synergy of the Dun with the UU in niche games. However, the problem is that it does not do as advertised in giving Guerrilla to all units. So around the time you are building GWs only archers get the benefit. That can still be interesting and I've had fun with it, but I'd prefer that axe/spears got the promo as well to support the GWs in a fast moving powerful army. Dun is by no means a good UB but it can be fun in the right circumstances.
Stele - At first glance, this UB is certainly not very synergistic with the creative Zara. However, it does have it's uses in securing borders or even commando settling against the AI. Still, not so hot a UU. I'd say one added bonus would be that the Stele never obsoletes. I think folks would be more inclined to build it. Otherwise, it is almost completely worthless and almost never built as Ethiopians except in some border cities.
Obelisk - Ha...change the priest slots to scientists, but then that might be consider OP. OR maybe 1 priest and 1 scientist. Kinda a mini-Madrassa. Heck, even 1 scientist only would be pretty sweet.
Assembly Plant - Change it to a Forge UB with extra engineer slots - it would also have trait synergy with 1 German leader. Germans have been around long enough to have an earlier UB so as not to have both late game uniques. Well, either extra engineer slots - probably not 4 though or a slight production boost. Not sure what a comparative historical building would be.
Pavilion - I'd rather have extra static happiness or health than extra culture.
Hippodrome - Keep the same, but return the artist slots. The building does have its uses.
Trading Post - Not really a bad UB in the sense that it replaces a building that you always build, but the bonus is not always that useful in some games (it is a nice bonus though and synergistic with the UU). I'd add +1 health or happiness to the building to make it have some value in all games to grow coastal cities a bit larger, which in turn can boost trade route income.
Citadel - Building can be a lot of fun if you make use of it. Like some other not so hot UBs it comes on a building that is rarely built. Furthermore, the building obsoletes rather quickly. However, if you do use it, you can build a bunch of siege quickly that you upgrade to powerful cannons later. IMO - I'd like to see the building simply obsolete a bit later to give a bit more time. Overall though, the main reason I don't like the Citadel is that the bonus affects units that are near in timeframe but very non-synergistic with what I think is the best UU in the game. Overall, I never build the Citadel though.
Forum - Nix the +25% GPP and add +2 health. For the only market UU, I'd rather see some synergy here that allows for more city growth.
Baray - I'm probably in the minority in thinking this is actually a pretty good UB. It provides a bonus on one of the most important aspects of the game - food. Yeah, +1 doesn't see like much but it still does a lot, allowing for faster city growth or being able to work that last mine or run an extra specialist when food comes out odd. Especially beneficial in those not so food rich resource cities you have to settle, making them more productive. And don't forget whip recovery either. When combined with arguably one of the best Leaders in the game - despite a terrible UU - it's pretty strong. Still, adding...say...+1 happy in addition to the existing +1 food to the building might convince folks that it is more worthwhile. Not bad for a rather early game building. I think it would put it more on the level of the excellent Hammam.
. For a building only hammer cities are going to have there isn't much synergy giving them a tiny % increase on virtually nothing.
to river tiles too.
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No, can't blame the UB for being on a sucky building. It's nice for the celt AIs.
Free march promotion? That would be a really good lategame building.
instead of 
would be interesting and not OP?
