An idea I've been toying with

[Southern Senator North Carolina Yankee]

Why not retain the 5 regional senators, and simply reduce the number of at-large senators? What's the reason to have the same number of each?

To prevent one from out voting the other. Basically the equivalent of the divide in real life between State based Senate and proportional based House, but in a unicameral format.

[Southern Senator North Carolina Yankee]

I prefer a bicameral system organized along the lines of the Duke Plan, but with a much smaller house than his 2013 plan called for. 

Original Duke Plan: Six Regional Senators (two per Region w/ 3 regions) + 11 I think member People's House.

So that but with like 5 members in the House so as to cope with our smaller active population now. That would be +1 on the national legislature, but a reduction overal of 6 to 9 seats from having fewer regions.

A Four Region map can achieve a sub 10 member National Legislature

4 Regions x 1 each = 4 SEnators  + 4 to 5 People's Representatives = 8 to 9.

But of course for those two fewer seats nationally, you might have three to five more regionally.