>If we develop a strong client with broad support, it makes developing the replication piece easier, since the client already supports the features in mind

Unless you are running the client against server, you will never know if the client features you are writing are any good.

The games need to run on the server by default, and the clients are just a "view" into what is happening on the server.

For ease of development, your app is a fully functioning server and a client in a single application.

When playing single player, the game is still running on a local server, with only a single client connected.

I strongly recommend building everything around the communication pipeline between the server and client. Make it very easy to dial up latency and packet loss.