@WNP78 I know you have looots of other stuff on your plate, with the upcoming patch... though I'm hoping that these may be small enough, but also worthwhile enough, that they may be doable still. 🥺 😊

I've tried to order these with prioritizing the simplicity of implementation first, and my desire to have it (ie its usefulness) second:
1) [Overload] - An Apply button.
REASON: To keep the Overload editor window open, saving lots of time when continually modifying something, while determine what works/looks best.
USE CASE: Lots of times I'm making changes to an attribute on a part that are either totally outside the value range of the vanilla menu system, OR, are in steps that are smaller than the vanilla menu system. Having to "Select part, Open Overload, Select Category, Modify Value, Click OK. REPEAT" eats up a surprising amount of time.
BONUS BUG?: Not sure if this is legitimately a bug, and even then if it's specific to Overload or if it's more a core SP thing... But I find that using Overload a LOT, tends to eventually result in Overload taking considerable time to open/switch categories. Not sure if it is a garbage collection sort of thing where the windows are thought to be open still, and editing more and more causes a bit of an unseen menu overload? Either way, clicking Play or exiting the Designed back to the main menu solves whatever causes that.

2) [Overload / FineTuner] Having the changes made with them be tracked by the vanilla "Undo" system.
REASON: Making changes primarily using these two mods leaves me in many situations where I lose considerable work on account of doing CTRL+Z to undo something I disliked.
USE CASE/REPRODUCING ISSUE: Say I'm making adjustments to a Structural Wing (aka Wing-2) I've just placed.
I. First I change its rotation (any is fine but lets say Y set to 15).
II. Then I open up Overload and go into the Wing category.
III. I want to allow Control Surfaces on it, so I set allowControlSurfaces: true.
IV. While I'm in there I make preliminary adjustments to its Root and Tip values: rootLeadingOffset: 0.85, rootTrailingOffset: 0.45, tipLeadingOffset: 0.6, tipPosition: 0,2.5,0
V. Click OK to apply the changes.
VI. Now maybe I want to add a Nose Cone, so I place one on the front of my plane.
VII. I decide I don't really want to use Nose Cone, but instead of Fuselage Cone... So I CTRL+Z to Undo
VIII. UGH... The Nose Cone is deleted as intended, but it also reverted the wing completely back to its default state: devoid of all the Overload and FineTuner adjustments I'd just made. Not only that, but it's randomly moved my build around in the editor window (usually towards Negative-Z and by a distance of roughly 4.0; sometimes more, sometimes less.)

#3 [FineTuner] MultiSelect: The ability to "Select All" before OR after making a selection.
REASON: Sometimes we want to do something that involves every piece of the build, And sometimes you want to leverage MultiSelect's ability to rotate around the first-selected part, and want the entire build to rotate around that specific (first-selected) part.
USE CASE: Perhaps wanting to rotate your build a lot, or just a few degrees. Maybe you want to re-center it because SP thinks centerline ought to be X: -0.10592 instead of 0.000. Sometimes you eventually decide your build needs to be at a slight incline. Or maybe building against a blueprint and its time to rotate it, where instead of having it rotate around the exact center, you select the first part to have it rotate around there, so that you don't have to bother re-centering it on the image. (granted, we can move the blueprint image, but nevertheless... heh)

#3b [FineTuner] MultiSelect: The ability to "Select All - Fore" and "Select All - Aft".
After making first selection, any part determined to be "in front of" or "in back of" the part, would also become selected. Could maybe use something like SHIFT+UpArrow/DownArrow after making the first selection, or simply have buttons in the UI for it.

#4 [FineTuner] MultiSelect: An option to "Create SubAssembly" out of the selected parts. [ALTERNATIVE: Being able to Copy/Paste what we have selected, bypassing the SubAssembly entirely]
REASON: It's possible I'm just too much of a noob and there's a simpler way I'm not aware of, but the SubAssembly (and Mirror) system lacks a bit of user-friendliness on account of the fact it relies on the Attach Points... Which is a mighty PITA.
Sometimes it's fine and I can use the Attachment Points menu to remove some, disconnecting a huge assembly from the main build, allowing me to then drag it all to make a SubAssem; however, that's still not very user-friendly. [Even then, not always do I really need to make it a SubAssem, only needing the complex arrangement duplicated in the moment; hence, the Copy/Paste being useful.]

#5 [FineTuner] The ability to DE-Select parts while using MultiSelect.
REASON: Sometimes we misclick and select a part we didn't want to. Usually this is not a big deal and we can cancel out, starting over. Though sometimes we've misclicked after having selected a vast number of parts and starting over is tedious!
I can understand that this is slightly complicated though, so even if there was new hotkey (CTRL+SHIFT+LMB?) to deselect the very last selection, that'd be beneficial. Or maybe up to the last 3 selections, should you not notice until having already made another selection.

"Stop with Five, Stay Alive!"
I started with 3 and came up with 3 more, so I'll stop before I come up with further ideas! :P

If anyone else finds they'd make use of this, by all means chime in. Particularly if you feel you don't agree with my ordering/prioritizing. If the consensus is it needs to be shuffled to a different rank, I'll be happy to edit it.

[NOTE: I believe that changes made using even the version of Fine Tuner that's built into the game -- that as far as I know was added by WNP78 -- also suffers from the aforementioned "Undo" issue. I just don't use that one since it isn't as easy. Plus, I figured making suggestions to the Mod version would have a much higher chance of being added since he doesn't have to worry about pushing out a full update to the game, and any major bugs as a result.
My point being, that I don't know if that happens because of a SP limitation, or if it's a wider-reaching bug... Because I DO know for a fact that it also impacts the Attachment Points system, meaning that 2 official menus have the same exact "forgetfulness" issue (their changes not being committed into the Undo-History)]