If the design didn't allow for dramatic improvements to things like engines, increased fuel capacity, armor, more weapons and various other factors, it wasn't such a good design. That is one of the main traits that defined a good aircraft in WW2.
In fact per the thread title, it's probably one of the less obvious but most important traits of a good warplane - the capacity for improvement and modification, as a subset of overall versatility. That is the difference between a lot of the other 1930's designs like the Curtiss Hawk, I-16, MS. 406 and the Hurricane: they were pretty good in their day, and remained formidable for a few years, but ultimately didn't have room for the dramatic improvements needed for the rapidly accelerating pace of aircraft design and the swiftly soaring requirements for an expanded flight envelope for the ever more dangerous combat environment of WW2.
The Curtiss Hawk was better than the other other three at being developed. It also shows a bit of the difference that just being a bit bigger can bring.
The I-16 was the first monoplane fighter with retracting landing gear and there was only so far it was going to go. It's first flight was nearly two years before the 109 so they are not really contemporaries. Installed power more than doubled over it's life but adding another 10-20% more power to it in 1941/42 wouldn't have changed much.
The Hawk on the other hand did pretty good for a plane that flew over 6 months before the 109 (in May 1935), unfortunately it was saddled with a prototype engine that went nowhere and Curtiss began a mad scramble to get a good engine for it. It started life at a bit over 4800lbs but that might not include guns and the P-40Qs hit 9000lbs so it certainly exhibited quite a bit of stretch. It was up against newer planes in way the 109 wasn't. The US aircraft industry came up with more and better replacements than the German aircraft industry could. In large part just due to the shear size of the US industry. But that meant the US could stop production of the Hawk in 1944 because it had better planes in large scale production. Had the US been facing a similar situation to Germany perhaps more work would have gone into fitting better engines in the last year or two of the Hawks life.
The 109 lasted as long as it did for several reasons, one was that the intended replacements didn't work. See 209-II and 309. Another was that in 1943/44 Germany could not afford the loss in production that switching to a new fighter type would entail. Another was the type of combat had changed. While the US (and the British) were finding short ranged fighters a bit of an embarrassment in the type of missions being flown in late 1943 and 44 a short ranged/short endurance fighter was still very useful to the Germans as the allies were delivering targets (bombers) practically to the German fighter fields doorsteps.
The 109 was still a dangerous opponent to the allies and useful to the luftwaffe but it was no longer in front ranks of fighter design.
A bigger plane allows changes to be made in an easier manner. Adding 100lb of pilot armor to a 4400hp plane means a bigger impact to performance that adding 100lbs of pilot armour to a 5500lb plane for instance. double in the weight of guns and ammo on plane with a 230sq ft wing is easier than doubling the weight of armament on plane with with 170-185sq ft of wing.
A number of planes used slats and a number of planes used fixed slots which perform essentially the same function to maintain aileron control at near stalling speeds.
notice fixed slot behind wing leading edge in line with the front of the blue circle of the national insignia. Lockheed 14s/Hudson's/Venturas and even some B-24s used such slots.
as did the Douglas SBD
Some STOL aircraft use a full span fixed slot. Please note that neither the slots or slats
increase wing lift on their own. They allow the wing to generate more lift by increasing the angle of attack without stalling. If the wing never exceeds an angle of attack of around 12-14 degrees (depending on airfoil and other things) a slot or slat does nothing except add a bit of weight and/or drag.
The use of slats/slots was a very common practice in the late 30s and early 40s but it was hardly an example of advanced thinking or design.