Replacing the pager inside vim?
Solution 1:
That pager is built-in: you can disable it with set nomore but you can't replace it with something else.
You'd better read :h pager and get comfortable with the mappings.
Also, what sophistication do you have in mind?
Solution 2:
That's built-in; you'd have to modify the source code, and recompile.
A radical change (or complete alternative) is unlikely to be accepted by Bram, but if you have good ideas for simple usability enhancements that can be implemented without introducing much additional code, please send such patches to the vim_dev mailing list.