When working with software and game development there is often times where the project will outlast the developers themselves - people get new jobs even move onto new things internally. We have even worked on projects with external teams where after a year of development we were working with an entire team of different people. This is pretty common and most people realize that from a developers perspective, although there is going to be a cost when adding new devs to existing projects and learning a new code base.
There is very little impact from a users perspective however. Maybe some of the updates are slower for a while but generally whatever the 'thing' is be it software or a game just continues to exist much as it did before. People shouldn't see a significant impact on the final product.
When a new Artist comes on the scene however it's a bit of a different story. The images that an artist creates will be right in front of a users eyes and if things don't quite match up it can go from being a little bit jarring to a complete mess.
When we had a new Artist start on a project that had been live for 5 years these were the types of things that helped us keep the product consistent.
Take Your Time to Study
Going through all of the repository from concept to final might be obvious but it's something that if overlooked can have significant impacts. If your project is huge you might not be able to cover every single image, but at the end of the day the repository is the one place where everything will exist so it's the best location to see all of the art in it's entirety.
Play the Game!
Again, seems like captain obvious speaking, but while you might not see everything while playing the game a few times, the critical thing is that this is ultimately how the player sees your stuff. All too often we see people make pretty art that looks great on a 24 inch monitor but throw it on a tiny mobile screen in a totally different context and it might not communicate the same.
Obviously it's not something you can just go back and time and implement but if you are doing a project and it's something that there is even a glimmer of hope that you might support it for a while then get a basic style guide from the start. When that original artist is long gone and you are trying to figure out how/why they did things a certain way it can be a nightmare. Our ultra simple style guides from 5 years ago were still totally relevant to our new artist which saved heaps of time.
Start Drawing Some Stuff
After doing the above we like to get a whole bunch of images on a second screen and just start concepting. One of the keys is self imposed time constraints. You don't want to make something ultra pretty and polished only to find a designer or a producer thinks it didn't hit the mark. We go for an iterative approach with ultra rough, fast sketches that we can get the direction approved without waste.
Some Differences Can Be Good
If it's something like a logo then you are going to have to be extremely consistent. But we have found when it comes to games people like a little bit of difference. It still needs to fit into the 'world' or the over all theme of the game but Artists should be encouraged to put their own creativity into something. After all that's what game crafting is all about.