I went this way back on 2017...
But after seeing the amount of work it needed, I decided to create a client with free assets, stepping out of commercial IPs. The result was Epic Dragon World.
I really hopped for more people to join, as with L2J, but it never actually happened.
Most people where there to rather criticize and project their own ideas and preferences, than actually help.
You may consider using it, to make make your project multiplayer and more appealing.
The necessary code to connect a Java server with a C# Unity client is already done and it is quite simple.
Server: https://github.com/PantelisAndrianakis/JMMOS
Client: https://github.com/PantelisAndrianakis/EpicDragonWorld
The exact same networking is used by all projects at L2jMobius.
https://bitbucket.org/MobiusDev/l2j_mobius/src/master/L2J_Mobius_CT_0_Interlude
It is possible to connect L2J_Mobius_CT_0_Interlude with a Unity client.