To have a "similar" course you need a city which can feasibly have a climb in the 4km @ 10% ballpark - usable as a pass - immediately overlooking it. That obviously hugely limits the options even before you start saying (understandably, by the way, since most of the péloton will be there and you have the issue of decamping the whole circus to another continent at present) that we're restricted to Europe. You immediately exclude the Netherlands, most of Belgium, 3/4 of Germany, most of Poland, Denmark, Sweden, Finland, the Baltic States and the majority of the UK (notwithstanding that the UK is behind the curve in opening back up it would seem at present and also held the Worlds last year so would seem fanciful as an option). You're left with a very small selection of Belgian towns that could have a climb in anything like that kind of ballpark, especially as I would venture to suggest that La Roche-en-Ardenne is too small to viably host the Worlds. Out of 'traditional' cycling countries you're left with Spain, France, Italy, Switzerland and a couple of fanciful options in Luxembourg. Out of less traditional countries I guess there are potential options in places like Austria, Czech Republic, Slovakia, Slovenia and Portugal, but after that you're getting into peripheral cycling nations - though I guess Romania has held a race already this season. Austria is probably less likely as they held the Worlds just two years ago too.
Regardless of the likelihood of these places ponying up to host, from a "produces a reasonable facsimile of the Martigny course" the immediate thought is Turin, with the Basilica di Supergà climb at
4,9km @ 9,1% and easily manageable on a circuit of reasonable enough length to use as the Worlds (plus plenty of flat land around there to do a direct copy). The other immediate climb to spring to mind is of course
Civiglio, 4,2km @ 9,7%, which we all know from Lombardia of course. Another good facsimile could be offered by Eibar, using the
Azitain side of the Alto de Arrate (really the Alto de Usartza which is about a kilometre above the sanctuary, but in cycling parlance it's always named after the sanctuary); this is the side climbed in the ITT a couple of years ago, not the
hormigón side climbed last year which would also fit from a gradients point of view but is a bit of a gimmick. Arrate via Azitain is 5km @ 9,6% with the main body of the climb being 4km @ 10,8%, so actually even tougher than Petite Forclaz.
It's probably no surprise to learn that País Vasco has a couple of other options for a course of similar fashion - Bermeo offers the classic Alto de Sollube - you could descend the traditional side, and climb the 4,2km @ 9,3%
Almike side - Aupa Urdaibai!!! Elsewhere there's
La Comella if you want to hold the Worlds in Andorra - 4,3km @ 8% and easily managing a circuit, though finding the flat first half of the course will be a chore! Mallorca offers
Sobremunt, as far as the junction for Esporles, in the village of Es Verger. You could have a flat first half around Palma de Mallorca, although to make the circuit the right length you probably can't have the finish in the city, and the asphalt is not in a particularly good state there. I think I know a couple of forumites who'd be pleased with
Cresta del Gallo being the key feature of a Worlds in Murcia, though I think it's not really tough enough to match up to the Martigny course.
In France, my first thought is of the
Col du Lançon which would suit a circuit around either Arreau or Saint-Lary-Soulan and with a last 4km at just under 10%. Mende would be the other logical option, with Montée Laurent Jalabert. Aue in the former East Germany would be good, with the 3km @ 10,7% Teufelstein, with proper Peace race legacy too. Wisła could offer something with Przełęcz Kubalonka as well. A really good one would be Liechtenstein, actually - you could climb the
Buchs side of Malbun as far as Triesenberg and then descend into Vaduz. That's at about 970m altitude, so about 5km up the ascent.
Of course, it'll be in Doha.