We can separate the possible solutions into intrinsic and extrinsic solutions, as well as sitting/standing solutions.
Intrinsic solutions (ones that will not look like an afterthought):
The 27" iMac weighs 21 pounds (9.54 kg). This could make it challenging to couple it with some random monitor arm on the market.
Extrinsic solutions (an additional base with a potentially jarring look):
The trouble here may be that you're designing a desk for just one person. If you are pursuing a more fluid ambiance at an office and wish to accommodate someone ranging from, say, 5ft to 6ft 2", the presence of a base may not be too convenient.
- Ikea sells a manual and an electric adjustable desk. They are sitting/standing desks, and so they are a much more general solution for problems in ergonomics. To use while sitting, you will need to look for a really deep keyboard tray (or possibly get a second lower table to position underneath, if you won't find that setup particularly jarring).
- Ergotron WorkFit-A, Sit-Stand Workstation, $595
- Varidesk standing desk Pro Plus, $395 to $595.
Note that neither the Ergotron nor the Varidesk solve the problem since the height difference between the keyboard and the monitor platform is constant. A monitor pedestal will still be needed.
To conclude, your best bet may be to look for a sit/stand desk (google/bing) and hook an adjustable keyboard tray (google/bing) underneath. Unfortunately that's not a cheap option.