Im also looking for new luggage atm and notice that the largeest bag in 3 bag sets tends to be be 76cm which is too large for domestic use; and the carry-on tends to be 55cm which only just sneaks in (my current carry-on is 48*34*23, which works for dash 8 which has a smaller requirement).
For domestic trips longer than 3 days I typically take my 66cm samsonite (140 linear cms) but when expanded comes it at 145 linear cms and havent ever had a problem with it being accepted in its expanded state. If i need more I just check a second bag (have been gold or qantas club so hasnt been an issue to check 2 if needed) or take my carry on case as well.
My samsonite is looking very battered so Im thinking of getting 2 * 66/67cms cases (these come in at exactly 140 linear cms) so are perfect for domestic and international, even it it will take a little more storage at home cause cant nest them. On the plus side the handles will come to the same height so will be easy to driver 2 bags with one hand.
Trying to decide whether it would be better to split clothes evenly between the two cases (so both half empty) outward bound or fill one and check one empty (not sure if that will look suspicious); as I know I will need two coming home via the USA.