What are some of the ways by which we can broaden successor pools during a talent review process? Also looking for insights around implementation.

I worry that the Talent Review process is too late to broaden successor pools.   The traditional view that someone is 'ready' for the next role is more than just a function of their own willpower, it's even more the result of the organization around them.   There are organizations out there where everyone leads and people are able to choose forselves what roles they are ready to take on.   This form of self management turn this talent review process on it's head and they have thee problem of too many candidates instead of too few.    I tried to summarize the practices of these companies and the mindsets of the people in them here...   https://www.linkedin.com/posts/charlierataj_the-empowerment-coin-activity-7227671767895457792-3asO?utm_source=share&utm_medium=member_desktop&rcm=ACoAAAGIPs4Bu9sTx7dLr7u3sYtBGCB9aUaqcNk

I'm sorry this isn't a direct answer to your question, but I want to invite you and others to look at problems like these systemically.  

A couple things we do:
1) Have lists of all the High Potential / High Performing Talent in that function to ensure no one is being overlooked.
2) Build a list of agile high-po talents you want to move x-functionally and ensure they are all on a plan outside their function
3) Ask leaders to think about how they could shape roles differently in the future and then see if that brings up additional talent for consideration.
4) X-functional succession plan reviews with your leadership team to surface additional ideas

A few things we have done include: senior leader forums whereby the executive get to see leaders perform and contribute from across the business.  Each executive mentors at least one person from a different business unit, thereby getting to know the strengths of others.  

Something we have in the pipeline is to create a skills platform to open up transparency of capability across the business, better enabling cross pollination across the business.  

Not sure if this is what you are looking for, but hopefully it is helpful in some way

One way we broadened the pool was to include leaders from other business units to participate in the review process so that they became aware of potential successors in other markets.  That may not work for you if you don't have a large or distributed business.  We combined our succession planning reviews with our strategic planning reviews so that leaders were linking their talent needs to their strategic plans.