I've thought about the same earlier and looked at each one. Without really remembering all the details I think the big picture was along these lines:
T-shank adapter: Fairly easy to do well and compact at that. Also a great variety of various of tools available - mainly saws, but also some files and knives. The main issue is that most of those tools are made to be used with a power tool. Few seem stiff enough for manual use, and the frequency one can achieve manually is far less than the power tools they are made for.
1/4" bit driver: Takes some width, and needs a center position on a generally straight tool to be used properly. Very flexible though with lots of inserts widely available. It is kind of useless though without bits, and the right ones at that, so those will take some space as well - and that space is not available inside of normal sized multi-tools. Still, a great option.
Utility blade holder: There are many utility blade standards but the most available is Stanleys. The holder can be made quite compact at about 4mm width, and again there is a varity of options for the blades themselves. It does make a lot of sense for say a urban work tool. The main issues it the height it requires which limits it as an option for high handles. Smaller blades than Stanleys is an option of course, but then less choice and not widely available.
Scalpel blades: I'll just add that one as I looked at that too, and it is in the same general direction as the others. Fairly compact holder with lots of cheap sharp blades available, and in several size standards too. Not very sturdy blades though (unlike utility blades), and not long enough blades for general stuff. Thus mostly for precision cutting. A multi-tool is also a rather clumsy handle compared to the thin sleek handles they are normally attached to.
Replaceable wire cutters: The main problem is that multi-tools don't have proper cutter geometry in the first place and thus get damaged easily. I'd prefer a solution with proper cutters that do the job, rather than fixes for when it couldn't. (That might be harsh as it will work for lots of things, but you get the idea).