I know with CAD/CAM that's probably changed a little bit, especially as the human element has been removed from the manufacturing process.
That said, let's talk gaps and very needed upgrades:
I deal in three primary areas: Russian Subjects, US Navy pos-war and then World War 2 (primarily the allies, but I'll build a Luftwaffe subject every now-and-again).
All should be 1:48
Russian Jets
MiG-31 - No, the Lindberg kit does NOT count, the "Evergreen" kit is a better starting point.
MiG-27 (although the Trumpeter MiG-23 has the starting for this) - No, the old ESCI kit does NOT count either.
Su-9/11 - So far only conversions (for the OEZ Su-7) have been available, but never a full tooling of these types.
Su-17/20/22 Early Series (the Sukhoi S.32s) - Never been done in 1:48
Su-34 - no tooling in 1:48, and this is in series
production now with a new batch having just been accepted by the VVS.
Yak-25/26/27/28 family - lots of variants, and at least two distinctly different "lines" - let's start with a Yak-28P, and at least fill one "Yak Twin" slot
Tu-128 - Yeah, it'd be big, but I'd still get one (this is one kit that probably lends itself to the non-mainstream - low-pressure, or even fiberglass type kits, HpH for example)
Badly needed upgrades:
Su-7 Family - Only the old OEZ (and derivative kits) is available, it is one version - the last, and somewhat uncommon version. Requires non-trivial surgery to make anything but the BKL, even though BMK wings are included.
Su-17/22 Late Series (Sukhoi S.52/54) - Again the basis is an OEZ mold (though never released by that brand). The detail is soft, the fit is POOR, requires a TON of aftermarket to get it anywhere near acceptable.
Su-25 Family - The Revell kit is based on a prototype, and even then has a number of "assumptions" and inaccuracies built in. The alternative, again, is an OEZ kit - that like it's brothers above, just doesn't stand up. Fortunately a simple re-scale of the Trumpeter kit would go a LONG way to filling this gap (yes, a few corrections would be nice to see added)
Nice to have upgrades:
MiG-15 - the Tamiya kit is showing its age, and while the exposed engine is nice, it's tough to assemble the whole thing, especially for a NMF. Just a two part fuselage would be great to see. Also seeing a straight MiG-15 would
be most welcome
MiG-17 - The Hobby Boss kit is similarly flawed as the Tamiya having a four part fuselage. And the quality of the molding isn't as good. Considering this started life as the highly flawed VERY early Trumpeter 1:32 MiG-17, the kit is loaded with compromises.
Su-27 Later Derivatives - those that have actually made it to production would be nice to see. The upcoming Hobby Boss Su-27 may have provisions for this.
US Navy Jets
F2H-1/2/2N/2P Series - Seriously folks, this was an important combat type, some of which were flow by some VERY famous aviators (Jim Lovell anyone?). Why we don't have one of these is BEYOND Me.
F2H-3/4 - Different enough to merit its own line item. Those that follow famous USN fighter units (VF-31 Tomcatters for one example) would like to have this one to fill the "unit history gap." These flew in glossy sea blue, silver and gull gray over white - so there's a variety of schemes too (and, our north of the border brothers flew these for years as well).
F11F - Not that long in the fleet, but a very attractive jet regardless - AND, until the F-18s broke the record, it was the longest serving type with the Blue Angels.
FJ-2/3 - No, these are not just F-86Es with tailhooks. Too many differences to enumerate here. Again, a variety of schemes, USN and USMC service. Common tooling gets 95% for both variants - just need a different intake a nose piece for the FJ-3.
AM-1 Mauler - and you though the Skyraider was big
AF-1 Guardian - The hunter with the belly radome is kinda ugly, but the killer is pretty nice looking.
World War II
I did the Soviet Jets Above, but that didn't mean I was done with Soviet/Russian Subjects:
Yak-9 - SERIOUSLY folks, why is the ICM kit the best offering for this highly produced and widely used WW2 fighter? Heck this merits about four different lines, early series, mid series (T/K/DD), late series, Korean War variants..... They made a zillion of these things, but we keep getting Yak-3s? C'mon folks, this is a HUGE WW2 gap!
LaGG-3 Late Series - ICM did a nice job on the early series LaGG-3s, and appear to have provisioned for the later series, but so far as I can tell never followed through with those versions.
Pe-2/3 Family - The MPM and HiPM kits are decent, but this was a very mainstream, widely produced and deployed subject. Seems like a hard-tooled version of this would be a natural. Not all that big either. With the Tu-2 gap finally filled and the Il-4 coming, this one seems like a natural.
Tempest Family - one of those that always seemed like Hasegawa would get to it when they were on their WW2 run.
F2A Family - only low pressure kits and the old raised panel line Tamiya kit, seems like a hard tool version of this would make sense.
I'm sure there's others that I'm forgetting but hopefully that will keep the discussion going.