Thought I saw this before, but maybe it was not proposed as an idea, but…
“First to complete badge”, and perhaps also badges for who was 2nd, 3rd, 4th, etc. Just finished my 2nd city, St. Pete Beach, and was the first to do it. Also was first to complete Keller, TX, but that one is waiting for the next OSM update to show 100%.
I don’t think this needs saying, but I will. With OSM updates in operation now, having the completion date along with the badge will be important. We live in a changing world, an while I’m now 100% in two cities, I don’t expect that to last, especially in Keller.
So, with the date, perhaps there might even be an OSM query to show what streets had been run, up to that date?
As an FYI, I am content that I finished all the streets, that existed in Keller, when I lived in Keller.
I just want to echo what Eric has suggested. I think being the first to complete a city is a definate achievement worth celebrating. However, I’m wondering if there should be some city size cut off? Looking at my profile at most completed cities:
My target is to complete Bracknell with 1560 streets. To be the first to do so is, in my opinion, worth an achievement. But to get the same achievement for the five streets in Aber Community seems wrong.
Cheers,
Paul
Aber sounds awesome… But you raise an interesting point… Size. But there’s more than just a count of streets. There is also the number of miles (or Ks) it takes, or took, to collect all the nodes. So I personally don’t have a problem with someone being the first to complete a LSC (Low Streetcount City).
In fact I’m working on Lipan, Texas - CityStrides a massive town in Texas of 22 streets. It’s close to where I’m about to move… Granbury, Texas - CityStrides and thought it be fun to 100% another place.
There’s an odd aspect to this…
PersonA is present in CityStrides today & completes CityA tomorrow. PersonB joins CityStrides next week, but they completed CityA last month and 
didn’t know about CityStrides while they were doing it.
From what I’ve learned in building CityStrides so far, people do not react positively to changes in “truth” (with good reason). So, I couldn’t really “move” the badge to PersonB because then I’d have to handle support requests from PersonA (multiplied by the 60k+ active cities, that’s a new full time job 
).
Though… If I can clearly document that it is not a permanent badge, then that could work.
The badge “First to complete on CityStrides” becomes true and constant, regardless of someone subsequently joining who’d already done all the streets. It also remains true if the city changes and adds some new streets. The original person to get the badge still remains the first to have done so on citystrides. So no need to make it a non permanent badge.
Maybe we should take it down a notch. I think the main issue is with the concept of who was ‘First’. If instead you get a batch for completing a city that stays even if more streets are added (maybe with a date), that might already be nice. It is still weird though, because if more streets are added and you complete the city again, should you get a second badge?
How about having a date completed for a city instead like when one completes a street. A completed date would be accurate since all runs logged by various trackers are dated. If one joins late, but has completed a city once all their runs have synced, it should give accurate date of completion.
I think of this as, “The person who, upon processing, is the first to have gotten all the streets, is the first person.”, and they get the badge. If the city changes, it alters nothing, at the time (of processing) someone was still first, for that place in time. And now there can be a 2nd, 3rd, etc.
I do like @dljamet_1999 's idea of showing the date completed too!
I have a few thoughts to bridge what James said: could we create two tabs on the current City list that appears on the strider Profile page that mirrors the “Complete/Incomplete” listing of streets on a city or activity page? That way one can easily see what cities are 100%ed. Since by definition the tab would have only completed cities, instead of saying “100% of X streets” below the purple bar, perhaps it could say “X Streets, completed on [date]” where [date] = date of activity that completed the last street. Could even go crazy and have that date link to the activity that polished off the last road[s].
It’s not a badge but it is a trophy case of sorts!
With subsequent OSM updates new streets could be added that no one has yet run, so completion badges for 1st/2nd/3rd really will be in flux and people could get upset if they are re-ranked, as we’ve seen with the at times semi-contentious debates on what is or isn’t a street. But if the 100%ers for a given city are ranked (& re-ranked) by date completed, it’s not like their gold medal is stripped from them or something, it’s just a fact of when the activity was that completed the city.
Finally, a plug for my (so far) less popular ideas that bear some overlap with this: More Striders list metrics
Edit: oh and one more thing, having separate tabs of completed and incomplete cities on the Profile page will make it more practical/easier to use - a lot of the “cities” I’m currently working on are on the second page or later of my Profile page Cities list.
I had proposed a “pinning cities” idea so that a runner could have cities they are currently running on their first profile page (maybe three to five cities). This could be helpful for runners who have multiple pages of cities with various completion percentage that they have ran, but are currently working on one or two with a low percentage and having to find them. This could also be used to display a particular city that one maybe proud of completing on the their 1st profile page since cities are listed by percentage complete and then alphabetical order.
We just had that discussion totally unrelated at the end of a run yesterday! A couple of running buddies and I have been venturing out to surrounding small towns to complete them lately and our first pages are filling up.
I found this idea and went and added to to and voted:
We had both a “tabs for completed cities” idea and a "check box to hide 100% cities from the profile page list idea in the group
Thinking about it a bit more, and pairing it with Adding date complete to city and Badges for milestones and the fact that a city update can change the city in a way that changes your percentage from 100% to something lower …
It’s been a while since the badge/achievement idea went up, but I am still actively thinking about it & I expect one of the first badges I release will be a “Completed [city name]” badge. This way you can earn it, the city can change which decreases your percentage, but you retain the fact that you did at one time complete it.
This could also help fix up a bug where you complete a city … have it connected to Twitter so you tweet about it … the city gets updated in a way that adds nodes … your account is reprocessed and those nodes are already completed by past acitivties … which marks the city as complete again … and sends off the tweet again. 
Long-term/future additions could include a first/second/third place that can shuffle around if someone else provably finished it before the current first-place-holder.
Tht approach is what I am preaching for years now. When one has 100,0%, it is an official completion ‘at that point in time and history’. It could/should be rewarded.
I so hope this Badges will be processed with data from the past. Wouldn’t want to miss all my ‘1st to reach 100%’ on a lot of cities here! 
One downside I see is when cities or towns are imported in an incomplete fashion in the first place. Probably a lesser concern for larger cities, but it really depends on the variability of how well or poorly it’s been curated by the OSM community. In the case of the more rural part of Portugal I spend a lot of time in, there were only 2-3 roads per village before I started updating them myself…but even in the metro area of the capital Lisbon, there were big sections missing road names! I still do a lot of updating every week/month as I expand my range on exploratory long runs.
tl;dr some OSM updates are not for new construction/development, but due to incomplete curation in the first place.
Apologies if this is a repeat. How about a badge on a users profile if they are the very first to complete a city. More than one person can get a 100%, but for the first to complete, there can be only one.
I think that the First, second, third, etc. city completion badge topic is what you are looking for 
Perfect. Thanks.
Actually. When you are in your profile and you view a city and then view other striders who have hit the city, maybe pin the first finisher to the top with an identifier badge there. 1st 2nd 3rd. A hall of fame.