I tried to address this from the stand point of what I still own that's not Festool, and that I use frequently. The belt sander is a no brainer. My best flat work unit is m Porter Cable locomotive. It's an antique worm drive pig that will destroy a project in a second of inattention, but once you get the hang of it there's nothing plastic and new that comes close IMO. For quick and light I'm constantly grabbing my Bosch 3x21 or 4x24. The Multi-tool thing is cool but my Fein sits for weeks at a time without being looked at.
As for recip saws, although I don't do field work or many installs any longer, mainly just in shop, my Porter Cable Tiger Cub I think it's called is slick as seal snot. The articulations allow it to go places that nothing else will fit. A Festool variant that incorporated a brushless E.C. - lighter duty motor, cordless w/ LiOn batteries, and some sort of off-set attachment that gave it the flush cutting capability of my Bosch fine cut 1640VS while keeping the articulation of the Porter Cable. Don't care for their blade changer though. It's very positive and Ive had better longevity from it than the DeWalt/Rigid style but getting a hot blade out is a test of your tolerance for pain.
How about a panel cart that worked with everything. Something that lifted panels from vert. to horiz. and visa-versa, and/or fed them onto my MFT (I use the old profile on my bench with 2 meter rails). The panel cart would also of course be height adjustable, handle multiple sheets, have locking wheels actuated by a single stomp down pedal, also adapt to hauling a mess of systainers into a job, and maybe even work as its own cut table or Kapex stand. Oh yeah, and when not in use it would fold up and store in a Systainer I.
The impact driver would be awesome. I brutalized my little Bosch LiOn compact for several years before finally wearing it out. It's repairable but a new one costs about the same and my Festool cordless drills do just about everything else that it did just as well...just a few spots where the impact is still a nice go to. The Rigid Rt. Angle junk I burned up two in the same day, (never, ever, never would I ever buy another tool of theirs). Something of Festool quality w/ the Bosch size and function and nothing in common w/ the Rigid.
A larger adjustable bed for the 850 planer to convert it to a portable joiner. Some mechanism for thickness planing would also be trick.
The mini CNC idea is cool, but I tremble at the thought of the price.
How about a band saw. Since ditching my table saw and going all Festool in my new closet sized shop, my Mini Max, thickness plane, joiner, drill presses and lathe are the non-Festool gear I have left other than some sanding equipment. Festool needs to get on the ball or send me some friggin' green paint.
What about a Systainer based HVLP finishing system?
A larger dust evac system that would adapt to a central design? (or handle my planer)
A portable dust collector/air cleaner for the airborne bits that aren't already gathered up.
A lay flat tape measure with metric/std. markings.
Something smarter than the pocket multi-tool torx/hex set (it always makes me curse).
A combo square set like my Starretts', but something that won't make my heart stop if it gets dropped or knocked on the floor.
How about a multi-spindled head that would allow my 2200 to drill five or seven shelf pin holes at once instead of one at a time with the 1010.
A hard case for storing/transporting guide rails.
An offset base to trim right up to a wall, and a seaming base for the 700 so I can sell my old Bosch installers kit.
Low draw LED flashlights that use existing batteries (this is my personal crusade - all manufacturers should be forced to adopt a standard battery and stick with it for the next decade - this battery scam is drivin' me the short distance that I had left to be totally nuts). The flash light could extend and articulate, and have a couple of different attachment bases (magnetic, clamping, etc.) that would allow it to be used on a tool or rooting around the back of a dark work truck.
How about a convertible step ladder/stool/work stand that folds and fits with the Systainers?
Modular half height Sysports that could integrate with the Basic System modules and maybe different MFT profile rails to create custom benches/work areas. Work areas could be tied together to allow pieces to slide from station to station (alright maybe that's a little out there).
Systainer job site sound system w/ multi-charger, I-Pod dock, power distribution w/ GFCI, etc.
Panel/box clamping system that allows stacking multiple glue-ups and efficient clamp storage.
Sell the articulating arms here in the States (as long as you bring the belt sander).
O.K., that's about enough fantasizing. Back to reality. My local dealer had none of the vacuum or jig saw accessories that I needed yesterday, so it's back to making due with something less.
Tomorrow's project...adapting a Kegorator to work with my Systainers.
Oh yeah...I did read somebody's idea about an over arm or overhead type router. How about some kind of improved radial arm saw type set up?