Cool ideas? I don't know about that, but I have plenty of boring ideas. I'm sure there are many other options out there, but here's a few basic ones to get you started.
1) Do not ever use character talents. See how far you can go at level 1.
2) Do not ever use officers. Combine with #1 and see how well you can survive the redacted!
3) Monofleets. Pick one ship, and build your fleet entirely out of those ships. For higher difficulty this means you don't get logistic ships. For a softer difficulty you can use tankers and cargo vessels, but they obviously aren't allowed to fight.
4) Tech tier themes. Only pick low tech ships, mid tech ships or high tech ships. It's a more flexible variant of the monofleet.
5) Restrict weapons. Don't use missiles, or don't use torpedoes, don't use long range weapons, or something like that.