On: 0AD, an open source historical RTS in development ...

[philipswood]

There is a "deathmatch" mode you can setup during map selection where all players start with ample resources and you can enforce a starting timed ceasefire to allow some building before the battle starts.

[heavyset_go → philipswood]

The deathmatch mode also literally gives each player 50k in all resources to start out with.