Instead of adding new, isolated tech trees like the most recent additions (Poland, Sweden, Italy), why not start adding new branches to the existing trees? I personally enjoy repurchasing an old tank to go down a new line much more than starting a new T1-2 and playing a whole new line with only one playstyle (looking at you Italy). Being able to pivot from between tank classes midway through a grind is much less frustrating than losing all your progress and starting another line.
Pushing further, I think WG shouldn't be so afraid to overlap countries within the same tree. For example, if there was one "Commonwealth" tree with a bunch of Canadian, Australian, etc tanks, it would serve the same purpose as multiple sparse tech trees, without insulating each new batch of content. This also wouldn't break the "historical" aspect to progression, since you could tie it all into the history of each tank line's development (as a Canadian I know we basically rented our War Machine from those nearby).