if you have a look at all games from more than ten years, a global tech tree is quite always provided
I think it's a common mistake. Take for example "civilization". In your first game, even with the complete tech tree, you don't really know what is the better path, because you lack of real clues.
After you finish the game many time and begun to found it too easy, you'll read strategy guides (or made your own) and after that, the tech tree has lost all point because you perfectly know what is usefull and what is not.
And this is why I say it's a mistake : when you discover the game you don't need it because there is so much to discover, and when you are a veteran, you don't need it no more = tech complete tech tree is useless in both of cases.
But you lack clues for help you to make the right choice, in your first game. In civ's, this has been partially solve by "advisors", which give you advice for what next tech you should discover in function of some flavor (military advisor or economic, etc.). But it is not sufficiant because the advisor don't tell you why you should discover this before that.
In openXcom, we have the vanilla's model :
- nearly no hided tech, and the only "tech path" is clearly explain in ufopedia : "you need a leader", or "you need a commander".
And the TFTD's :
- some hided tech but with no clue. You must research "depp one" to be able to search aquaplastic, but nowhere it is said, and it is not even logical. The same for vibrolam which need calcinite autopsie, but why ? you could be stoped in research tree but without any way to know how to restart.
The right path for me is between this two. You need to have as much clues in the game to avoid the need of searching elsewhere informations in wiki or else, but, you should don't have all informations, because it will break the mysterie and discovery part (and was boring for those who don't even read when there is too much informations).
And for the time the research will consume, the player will quickly be informed of that, by begining it. After few days, the progression show him if it will be a short research or not.