The 2020 remake rebuilt the first two games in Unreal Engine 4. Unreal games are relatively approachable to mod: assets ship in .pak archives that the community can unpack, edit and load as overrides.
THPSPro
The best-known mod is THPSPro. It brings in levels from across the series, rebuilt for the remake: THPS3 and THPS4, THUG 1 and 2, the Project 8 world with Downhill Rio, and some American Wasteland levels. It also adds custom restarts, a free-roam camera, unlocked hidden cheats, classic skins, and board and griptape graphics. Planned features include custom goals, online multiplayer and better park creation.
The same team also brought THPSPro to THPS 3 + 4.