That this is finally happening has spurred me to starting this post, which will by no means be completed when first published, I intend to keep adding to it over time, and suggestions in the comments are welcomed.
There are already a few "alternative models"-style blogposts and forum threads out there, for WHFB and Mordheim, and they can be great resources, but all the ones I've found tend to be quite all-encompassing, by which I mean they tend to include any model that could potentially be used for the game, whereas I am primarily concerned with finding models that fit the setting, ie they look like they belong in Mordheim/Warhammer's aesthetic. And that is what this list is intended as - as AoS proceeds we will doubtless lose more of the models that fit within Fantasy's traditional aesthetic in favour of the more bombastic modern style, so I intend to catalogue all the miniatures I can find which fit my own obviously subjective criteria for a "proper" Warhammer/Mordheim model. The list will be divided by faction and any visually-distinctive subfaction(models which require additional conversion work to meet the standard will be noted as such). It will eventually include every currently-sold model from whatever source(including GW/FW) that I can find which fits aforementioned subjective criteria. Obviously given the purging will likely continue, I would recommend you pick up any GW models on this list that haven't been repackaged for AoS sooner rather than later.
Before we begin a brief but sadly necessary aside which will not be discussed further; if you're reading this post and find yourself itching to inform everyone that third-party model makers who make miniatures which are visually compatible with GW's various IPs(which are themselves pastiches of historical peoples and events, folklore & mythology, and "borrowed" concepts from other creative works) are "leeches making a living from other people's ideas" or any variation on that theme, please don't, it's not a PoV I'm interested in hearing about and any such comments will be moderated.
A final special note; any models marked "AoW" are from the Avatars of War range - very nice models, but I would advise buying them from a third-party retailer rather than direct from the AoW website, as they can sometimes take an extremely long time to ship.
Captain of the Empire - GW
Empire Officer - FW; Be aware that the models from this set are noticeably taller than GW plastics.
Malika von Ostwald - Hasslefree; Makes a fine female Hero character, or even an effeminate noble male Youngblood.
Altreich Engineer - Black Chapel
Altreich Captain - Black Chapel
Landsknecht 03 - Brother Vinni's
Empire Greatswords - GW; A fantastic if pricey kit that can be mixed with arms & heads(avoid the bodies unless you have a specific conversion in mind IMO) from the State Troops plastics also from GW to make Averlanders or fancier Reiklanders warbands, or Heroes for other Human Mercs subfactions.
Kalyna - Hasslefree; A solid female Youngblood model, though myself I'd choose to make her top complete again.
Wulfland Captain - Black Chapel
Wulfland Veteran - Black Chapel
Wulfland Honour Guard - Black Chapel
Human Male Barbarian - Otherworld Miniatures; Give him a halberd and a more appropriate helmet and this chap makes a very fine Middenheimer.
Empire Huntsmen - GW; With some admittedly hefty conversion work to add breastplates and leg armour, many of the models from this plastic kit can be made into dead-ringers for the original metal Marienburger Hero models.
Tradkarl the Wanderer - Red Box Games; Note - very tall.
In practice, these two groups are too politically fractured and aesthetically intermixed to be separated easily, especially in the Mordheim era - a model with combed morion helmet could as easily be a Tilean Condottieri as an Estalian Conquistador.
Major-General of Lusia - AoW
Human Female Monk - Otherworld Miniatures
Arabic Corsairs - Brother Vinni's
Witch Hunter - GW
Warrior Priest(Hammer) - GW
Warrior Priest(Great Weapon) - GW
Flagellants - GW; Can be used as-is, but can be made to better suit the starved/crazed look of the Mordheim art by covering the exposed chests with robe and by using arms from the plastic Ghoul kit also from GW.
Drune-Hounds - Mierce; By not gluing on the horns and using a touch of putty to smooth out the skin slightly these guys will make a fine set of Warhounds - can be bought individually or as a pack of 5(inc 2 repeat sculpts).
Sisters of Sigmar
Sisters of Serens - Completed IndieGoGo; The presence of these SoS models here may be temporary as the producer is only taking orders via email(details in the IGG update, scroll down for English) until present stock is run down. If they return later I'll put them back up here.
Dwarf Treasure Hunters
Dwarf Command Set - FW; A tad hunchbacked for my liking, but still very nice details.
Cult of the Possessed
Empire Huntsmen - GW; A second appearance for this kit, which can also, with a bit of work, be converted into very nice Brethren with a fair resemblance to the original Mordheim models.
Warper Alpha - Quantum Gothic; This one is for those who prefer something more akin to the original metal Possessed in style.
Beastlord - GW
Great Bray-Shaman - GW
Aanchuth the Cursed - Mierce; Again, basic iconography removal.
Baagath - Mierce; Baagath and the rest of his unit would make solid Bestigor alternatives.
Gors - GW; Can also make Bestigors with a little conversion work, saving having to buy the 10-man kit for those when you only need 2, as well as a Centigor with the addition of a plastic horse and some conversion work, an even more efficient choice as you only need a single one of those.
Bloodbound Slaughterpriest - GW; Can be converted into a very imposing Leader or Hero model.
Marauder Warlord - AoW
Bloodbound Bloodreavers - GW; A typically overwrought name for a modern GW/AoS kit, the models themselves make a solid basis for converting Norse or Kurgan tribesmen(making the armour less ornate/more primitive and adding heads from the Marauder Horsemen kit in the former case, and removing unwanted Khornate iconography and using helmeted heads in the latter) - better Marauders than the actual Marauders kit.
Daemonsmiths - FW; You get more than you need, but the spares can do duty as regular Heroes, a driver for the Engine of Chaos and so forth.
Swamp Folk Warrior - Troll Outpost; Not a huge amount of choice out there right now for these gents, indeed this model is the only one I know of other than the FW ones.
Fimir Warriors - FW
Skaven Warlord - GW
Blunt-Claw - Mierce; He's a big fat hairless ratman, what more could you want?
Vermin Clan Lord - AoW
Skaven Clanrats - GW; A nice wee kit, with models inside that can fit multiple different Clan aesthetics. With some conversion work, will get you almost a complete warband.
Skaven Assassin - GW
Skaven Assassin - GW; One for those who prefer the oldschool Skaven look.
Arch-Warlock - GW
Packmaster - GW
Plague Furnace - GW; A pricey option if you can't find the parts separately, but the character model is fantastic and makes a wonderful Plague Priest, and the 3 crew can be turned into Plague Monks with some effort.
Vampire Lord - GW
Wight King - GW; OK, Krell himself is maybe a bit much for Mordheim, but it's such a lovely model and you can just about get away with using him as a Grave Guard(Hero Wight) from the BTB supplement Restless Dead warband.
Necromancer - AoW; His staff top could do with being toned down a touch, but otherwise a nice model.
Ghouls - Otherworld Miniatures; These older-style Ghouls are actually a better fit for the Grimm Fairy Tales-meets-Hammer Horror theme of the Sylvanian Undead than the new plastic kit IMO.
Ghouls - Heresy Miniatures; Available in the deal shown below or in smaller groups, another variation on the older style of Ghoul.
Magda from Afar - Red Box Games; Note - Slighter build than GW figures, but possibly the best "Gypsy" style model out there.
Akos the Scorned - Hasslefree; Reworking the legs with putty to have trousers and rough leather boots would turn Akos here into an admirable Domnu.
Sea-Dog Rifleman - Privateer Press; Note - taller than GW-sized models.
Ghasts - Otherworld Miniatures; The chap on the left there would be a good way to represent a Hero who's been made a swain/get.
Orcs & Goblins
Orc Warboss - GW
Troll - GW
Goblins - Crooked Claw Miniatures; A really wonderful range of Kev Adams-sculpted Gobbos, in active development as well so even more variety should be available in the coming months.
Skink Starpriest - GW
Black Ark Fleetmaster - GW; Makes a good Highborn.
Ogre Maneaters - GW; GW make several different Maneaters in resin and a big selection of Ogres in plastic, but the change in aesthetic from your more classical fantasy Ogre to the newer near-East-inspired look put some people off, best to browse yourself and see which if any you like.
Monk with Shield - Black Chapel; An alternative, slightly less ornate Priest of Ulric, or perhaps a Priest of Taal after a weapon swap.
Kayazy Assassin Underboss - Privateer Press; Another Merchant, this time of a Kislevite persuasion.
Mule-handler v1 - Otherworld Miniatures; Perfect for a BTB Muleskinner.
Necromancer - Mad Puppet Miniatures; Contrary to the name, I'm recommending this for duty as a Warlock, it evokes the same kind of grimy-weirdness as the original miniature IMO.
Cirlean Murmillo - AoW; A solid Tilean Pit Fighter model, could perhaps do with being made slightly less ostentatious.
Master Pathfinder - AoW; What it says on the tin.
Berserker Lord - AoW; Another alternative Troll Slayer from AoW, in case you'd prefer a different pose to the ones shown in the Dwarf section above.
More to come...
Removed Wulfhere and Cynemon - Models far too small relative to GW, both in stature and proportions.
Moved Drune-Hounds from Marauders to WH - Models too small for Chaos Hounds, but ideal size for Warhounds.
Added Crooked Claw Goblins.