If you want to add some replayability, I'd suggest you try Bob's mods. Other than that, I share the same feeling, low replayability contrasted by its high playability.
Space Exploration mod adds a wonderful "new game plus" type experience: launching the rocket is just the first step towards building a galactic empire. You actually ride the rocket up into orbit and there are many new space-only buildings and sciences.
It's amazing how powerful the mod system is. FNEI is a great in-game directory of resources/recipes that I'd recommend with any new mods you're trying. SpaceEx add a whole new 'navigation satellite' mode that lets your remotely configure buildings and an interface for selecting planets and stars (in addition to the standard new buildings/recipes/vehicles of most mods).
Factorissimo (building that can be entered and contain more land than their footprint) and Recursive blueprints (automated blueprint usage) also add tons of potential depth.
