Personally i do not see a reason that a reform to the USSR and the Italian focus tree would forfeit each other. Therefore i can very well imagine both countries getting new trees within one expansion.
As of yet, each expansion featured several big focus trees and without a doubt both mentioned majors deserve one. Also doing them would mean a huge leap forward for all players as those majors generally have a big impact on the course of the game: To produce a historical immersive game while AI is part of the matter, an (initially weak) USSR should be able to make a comeback and Italy should run the risk of losing stability if certain conditions are being met. In the latter case we also do not have the game being able to produce the historical multi-belligerent situation with the RSI. When it comes to minor countries, i'd consider Finland, Turkey and Greece as the most requested minors to receive custom focus trees.
A main foundation ingame mechanic that requires an overhaul is the surrender / peace deal mechanic. An overhaul would mean special rules / conditions that create specific outcomes. This would make sense for both countries - Italy becoming unstable after some severe military blows and the USSR possibly sueing for a bitter peace as we had it in prior incarnations of HoI. Along the way, the effect of countries changing sides like Finland, Rumania and Bulgaria as most prominent examples next to Italy could be handled as well.
While the addition of new decisions could create the desired (or even required) ingame effects as well, the problem of postwar border gore and the course of peace conferences in general often create frustration or even ire to players after an exhausting campaign they invested so much time in... To get some kind of logics into this and therefore more reasonable outcomes in general, the mechanic needs a keen hand and effort on the part of the devs.
Touching land war would help all powers and all theatres of war. But I'd not say that land war is totally off in general in the current state. The mechanics do work and they do it on a logical basis. However, AI definitely requires some scripts (one for each major) to produce a plausible amount of tanks and AT, too. As of yet, AT is useless outside of MP and major land offensives are driven by non-efficient INF on the part of AI. To get more interesting situations and a more challenging AI opposition, dev work must be invested. The required refinement would not necessarily be extraordinarly high. If the dev approach instead touches and changes the basic mechanics, the whole game will have to be changed as balance will totally go off otherwise.
If it comes to an equipment designer, i see a priority of a plane designer more than a tank designer as tech levels of planes get much more outclassed and thus have a greater impact than do outdated tanks in the game. Also, the phenomenon of many early development planes staying competitive throughout the whole war via upgrades can absolutely not yet be simulated.
This may sound like a wishlist and to some degree it is.
The game is about to finish it's fourth year after release. It could easily make another few due to the quality of game design and ingame content available and possible. But due to the severe limitations of the ancient game engine (graphics, game laaaags, OOS errors) it won't make it as long as an EU IV. I see room for several more expansions in terms of content but still believe that it only has 2-3 years left at best until the lack of performance will relegate it to the shelves for most now active players.
To win back former players into activity and attract new ones, the remaining DLCs have to be interesting / content heavy and appeal to many players at once. After the above mentioned changes the late war (among it the wunderwaffen stuff?) needs to be treated and maybe a case sensitive cold war scenario. More is to be imagined but given the mini-dev-team they have, i do not see much more coming within the next two years. Please do not forget that creating a new HoI V will require years of work without revenue and to me it seems the team really isn't big enough to do several tasks of such size at the same time.