I suggest you look at Lugia's draft racks and posts on the forum -- that will give you LOTS of ideas on what directions you could go. Lugia's racks are well designed, balanced, and a good use of HP. And his posts on rack designs are great.
The modular domain is SO wide open, IMO it takes having a concrete vision "this is my draft next rack, HP to work with, budget etc." and some iterations improving the design (with forum help) to go from an infinity of options to a next build that actually makes sense, sounds good, and is fun to use.
As far as actual modules go, I love complex oscillators (like Make Noise DPO and Instruo C-sl), wavefolders (like Intellijel Bifold) and power-filters (like Rossum Linneaus, Morpheus, or Intellijel Morgasmatron). Those types of modules embody what makes modular fun and interesting for me. So, I recommend you consider what types of modules are inspiring to you, and consider what type of build will support those. And remember, you'll need a good number of utilities to make a modular system work well! Balancing the modular system so it has a good proportion of module types -- this is key.
Good luck. Enjoy!