I'd love if i would be able to just build cargo/fluid wagons and then connect them from the builder to a train that I can drive manually, instead of having the need of always building a locomotive.

I suspect this would be a fairly straightforward change to make, but I don't have a computer I can run factorio on at the moment. If you are interested in modding you are welcome to make the change yourself/upload/update for version 1.0

Sadly I am not really into modding myself, but I'm looking forward to understand some things of modding too. Maybe it will happen in some time, till I have worked myself into it.