I don't mind modifier sets (more ISO, please!), but my preference would be for full sets. And by full I mean 104/5 key, plus ISO/Tsangan/whatever options. And a set of blanks to fill out those macro keys or specials on some boards.
I may not need a full set off the bat, and only want a set for a 83/87 key layout, but I don't want to be looking for matching numpad and mods down the road when I decide to switch the custom key set to a new full-size board.
Color options are cool, but I'd really like to see more vintage cap profiles offered in group buys. DSA, SA, KT, SS (okay, that last one is kind of strange...), something other than bone-stock DCS. If I'm going to get a custom set like Miami or CCnG, I don't really want to mix and match sets, I want boards with complete matching themes. I know it's probably my OCD talking, but filling in a 1.5 modifier spot with a 1.25 key just grates on my nerves!
Just wanted to add my two cents to the discussion.