Congratulations on your new case!
If you haven't bought the mutable clones yet - I would be seriously tempted to go for the originals
you have a lot more space in the mantis compared to your original rack - they don't cost a lot more in most cases and are significantly more ergonomic than a majority of the clones
there are 4-6 voices in there already (the plaits, rings, clouds clones and disting - 5 if you include maths - and 6 if you include the marbles clone!!!) - I would concentrate on modulation and utilities for supporting these modules rather than adding a 6th voice - links, kinks and shades make a great starter utility set for example - I'd definitely also want more envelope generators (get a quad - maybe zadar? and batumi) - and more vcas (veils is great) - and a better end of chain mixer (I use a tex-mix which is excellent value for money and expandable - and easily diy-able if you can solder) - maybe also a nice effects unit - fx aid xl is great value and decent ergonomically and modulation wise
re filling the case - that's what I thought when I bought my first case - 6 months later I bought a second case - 4.5 years later I have 1500hp with enough space free for my diy backlog!
"some of the best base-level info to remember can be found in Jim's sigfile" @Lugia
Utility modules are the dull polish that makes the shiny modules actually shine!!!
sound sources < sound modifiers < modulation sources < utilities