Why not? It'd defenitely add more replay value after completing them game. The first two should've featured something like this.
Imagine after finishing the game being able to select Lan Di or the Taijiquan master from Shenmue 2 and duke it out. DLC could be available too, allowing players add fighters from previous Shenmue games not featured in Shenmue 3. Heck even chars from VF or Yakuza could be brought over. No I don't think any of these characters should be playable in the main game. Just an extra vs mode feature. I actually prefer Shenmue's combat engine over pretty much all other fighting games.
Would be awesome. What do all of you think?