The re-release of DOOM II includes:

The Master Levels, 20 additional levels made by the community and supervised by the developers
Split-screen 4-player deathmatch
Split-screen 4-player co-op
Slayers Club members who purchase DOOM II and link their Bethesda.net account in-game will receive the Brown DOOM Marine Skin and DOOM II nameplate to use in DOOM Eternal.

Purchase of the re-release of DOOM II also includes the original DOOM II, first released in 1994.