guards already show up very close to, if not sooner than, the flame ht. guards are also more than adequate to kill a ht. 1 button or 1 nade and its dead. as for a non doctrinal answer, t2, mines, AT nades.
there will always be "less diversity" in the sense that some builds just dont work. to take an extreme example, no one will ever be able to pull off spamming only mortars. the reason is, its not a balanced army. if you build units that have an obvious weakness, you better build other units to protect that weakness. t1 is viable if you support it.
another example is assault grens. soviets counter them with m3s, so germans counter that with an upgunned 222. germans cant just spam assault grens only because they will get hard countered. if the german fails to built any AT, they lose. this is no different than building t1 and refusing to build some sort of AT. it would be nice if soviets could counter everything with t1, just like im sure germans would like a mobile counter to the m3 at t1 as well. i just dont see either of those happening, atleast not without restricting your doctrine choice.
Okay, guards and FHT can show up at approximately the same time, however guards are still not an adequate solution.
Let's continue with your examples. Obviously spamming a single unit is a bad idea. However with the current meta soviets can really only cost effectively purchase T1 or T2. Against assault grens you can build an M3 which counters them nicely, or you can build a maxim to suppress them - also counteracting them. Both T1 and T2 have tools to deal with assault grens that are non-doctrinal. However if I choose the M3 route to counter assault grens Ostheer still has options. If they built T1 they have gren squads. One faust from a gren squad is much more likely to be a death sentence for an M3 than an AT is to a FHT (which is fine). If the Ostheer did not build T1 and went straight to T2, they still have access to their AT gun - or 221 which is a mobile counter. They do have non-doctrinal options.
Basically this thread was to suggest another possible option of improvement to soviet T1 that is not doctrinal - one of the main reasons it seems to lack is because the FHT can run wild on them. I think everyone is well aware that soviet T1 is lacking.