My response is almost identical to Ben's. I 'collect' a lot of players but my primary PC is Billy Pierce. I consider the 'heart' of my Pierce collection to be his career-era cards. I include any card that identifies him..base, All Star, multi-player, etc from Topps and Bowman. Also included are cards from Post/Jello, Bazooka, Red Man, Exhibit and so on. It's more or less stuff that can be found on checklists.
To those I add career era team Picture Pak photos, promo and endorsement photos, post cards, display items, magazine covers, wire service photos, and so on.
And finally there are post-career items like modern stadium giveaways and, as Ben mentioned, the numerous modern reprints and such. I have a list of my Pierce stuff on my blog.
https://5toolcollector.blogspot.com/p/blog-page_9.html