DTM in the 80s ran the same cars as everyone else ( mostly Group A - BTCC ran those, the strange world that was european touring cars/the brief world touring cars also did, as did the aussie series iirc ). I don't know what happened to make DTM go the other way from BTCC on costs but we ended up with saloon cars filling the hole that was the lack of GT at the time, and a few years of awesomeness. BTCC went for the Supertouring formula instead, which was awesome in it's own way and actually just as fast as Group A.
Fast forward to 2000 - DTM had priced itself out of existence as the ITC several years before, teams had jumped into GT1 and promptly killed that, and Supertourers were now too expensive too, so what happens:
* DTM tries to carry on from where it left off, with formula of prototype cars shaped vaguely like executive saloon cars, only making everything spec in an attempt to keep prices down. If you look at a 2000 season DTM car it actually looks far nearer to a road car than the things we have now.
* BTCC keeps an open formula but tries to get people to build smaller, cheaper cars with some spec parts. This Super2000 spec ended up being used for rallying too, but again eventually ended up more expensive so got replaced by NGTC which is used now.
So what's the current state? BTCC is oversubscribed, and DTM has lost it's signature manufacturer. Subjectively I think DTM is a bit more of a spectacle - BTCC cars these days don't have the presence of a Supertourer let alone a 600bhp Group A car even if they're faster than both of them; there is a huge variety there though - but with teams clamouring to get in & just not having room it does seem the more successful one at present.
BTCC has the cars people are buying, too; you could argue that the Mercedes 190 was in DTM because every taxi driver in Europe had one but I don't think they're driving C-class these days
and the M series BMWs are once again GT cars. As a thought experiment for a formula, how about 4/5 door cars a size up from BTCC max size ( they're allowed slightly longer cars than TCR there ), with 1.6 turbo engines but hybrid powertrain? if LMP1-H proved anything it's the potential for a hybrid on a track. Nice fat tyres for big arches and decent grip, limited aero upgrades to keep price down & aero issues down.
If DTM won't, hopefully someone else will
