Firstly, although this project pursues the theme of "doom as in 1993/1994, but with more attention to detail (which I think not only I always wanted), but in the final variation and plot, i.e. without strong bible/alpha infusions", and I was not going to change the levels themselves as such, yet I think it will not be quite so.
Namely: E3M1, to be honest, always seemed too short to me, despite its impressive start. E3M9, in turn, was only an expanded copy of it. Therefore, there is the following idea: replace E3M1 with E3M9 (of course, with some minor changes, such as replacing the cyberdemon with a cyberbaron), move E2M9 to E3M9, and as E2M9 I plan to use E1M1 from alpha 0.5.
Secondly, I am thinking of giving the Doom 1 bosses more personality, even though this is corny. Of course, since they were originally conceived precisely as single bosses, in view of their replication in Doom 2, one could replace them in the last with weaker analogues, but I think most will not appreciate this, especially in view of their already established outlook in Doom 2.
Therefore, accordingly, new opponents will be introduced at the end of the episodes.
At the end of the first episode, there will be new guys called the Counts of Hell (Yes, here I decided not to use monsters already created by someone earlier):

They will of course have more health (most likely 2000 or 3000), and probably more powerful attacks.
As for Cyberdemon and Spider, I think to just give them more health, and equip them with BFG (again, it is necessary to check it in gameplay, so that it suddenly does not turn out to be too difficult).