A while ago I asked a quite related question. I received small but worthy feedback and also an overall feeling that maybe there is nobody here who actually tried it (or there is a silent one). So I am glad you asked "how would I...", not "how did I..".

I would:

1. Make a list of things you want to achieve. What does it mean to keep the team "fresh"? If there is practice or behaviour you want your team to introduce (or keep using), put it on the list. In order to bring it to mind, you can think of your process and every activity being taken. It helps to remind what are your thoughts about it (for example: daily meeting and what you want to change about it).
2. Make a list of rules or "boundary conditions". For example: "achievements should be clearly visible to anyone" or "gamification should not make the work worse for people who do not feel they like to participate" or "game elements should be interesting even for new team members". Remember about the rules and stick to them while designing the concept.
3. Consult the idea with the teams. They will probably come with new ideas for things they want to achieve (#1) and rules (#2). You can also get the feedback about your ideas and concepts (badges, XP's, levels, rewards etc.) that are already on your mind after creating #1 and #2.
4. Design the solution. Yeah, I know, it is easy to say. But it's too complex problem to describe it here. It's probably too complex even for a book, and yes, I don't fully get it either. It's not much, but here are some links: Gabe Zichermann talk and presentation he shows, and much more resources.
5. Try to hack the system. People like to take shortcuts and like to make achievements. The question is what will happen when they try to maximize their XP and will think only about game achievements not the real life ones. Ask the team how would they hack the system? Make necessary modifications and you are ready to try it...

It's my vision on how to make gamification work (very short version), although I am not sure if it helps keep your teams "fresh". There are many hidden pits one can fall into, as successful game design isn't easy.

A while ago I asked a quite related question. I received small but worthy feedback and also an overall feeling that maybe there is nobody here who actually tried it (or there is a silent one). So I am glad you asked "how would I...", not "how did I..".

I would:

1. Make a list of things you want to achieve. What does it mean to keep the team "fresh"? If there is practice or behaviour you want your team to introduce (or keep using), put it on the list. In order to bring it to mind, you can think of your process and every activity being taken. It helps to remind what are your thoughts about it (for example: daily meeting and what you want to change about it).
2. Make a list of rules or "boundary conditions". For example: "achievements should be clearly visible to anyone" or "gamification should not make the work worse for people who do not feel they like to participate" or "game elements should be interesting even for new team members". Remember about the rules and stick to them while designing the concept.
3. Consult the idea with the teams. They will probably come with new ideas for things they want to achieve (#1) and rules (#2). You can also get the feedback about your ideas and concepts (badges, XP's, levels, rewards etc.) that are already on your mind after creating #1 and #2.
4. Design the solution. Yeah, I know, it is easy to say. But it's too complex problem to describe it here. It's probably too complex even for a book, and yes, I don't fully get it either. It's not much, but here are some links: Gabe Zichermann talk and presentation he shows, and much more resources.
5. Try to hack the system. People like to take shortcuts and like to make achievements. The question is what will happen when they try to maximize their XP and will think only about game achievements not the real life ones. Ask the team how would they hack the system? Make necessary modifications and you are ready to try it...

It's my vision on how to make gamification work (very short version), although I am not sure if it helps keep your teams "fresh". There are many hidden pits one can fall into, as successful game design isn't easy.