Right now, in Original Skyrim, Destruction is useless both in early and late game due to high Magicka cost and weak damage. Will they make spells more viable again or will it be the exact same?

Opinions vary on the viability of destruction, but to answer your question, destruction won't change in SSE. That said, mods will be coming to consoles and a simple mod to make destruction more powerful/magicka efficient/scale better, etc. could easily be created with just the CK (no scripts or external assets needed), so this is a good example of the type of mod that could easily be made for both consoles, notwithstanding the limits on PS4 mods.

By everything Bethesda say is the same game as before, there is of course a realistic 5% chance they may be hiding some minor OR BIG changes as a surprise, so who knows?.

Frankly I don′t see a problem with vanilla destruction magic, apart from being kinda boring that is, I mean: Dual cast impact Fireball! *Stagger lock enemy*, *Stagger lock enemy*, *Stagger lock enemy*... Flawless victory!.

You can enchant and cast spells for free even without cheating or mods. The damage you do is controlled by the difficulty you play, so if you want to feel a badass and kill easier, lower the difficulty. This is a game for mature people, mature people create their own difficulty by weird self-imposed rules and don′t feel ashame of playing a game in the lower difficulty if that mean enjoying the game...

I mean, no one have to know if you do. I wont judge, I after all gonna play it in Expert :]. Legendary difficulty is for OP stealth hunchbacks who never fight face to face, because they will die. And thats unacceptable for the delicates egos xD.

Nothing related to gameplay is going to change for the vanilla game. As Turija notes, a mod can adjust Destruction spell damage.

Having said that, I disagree with the premise that Destruction is under-powered. I think it's more a matter of people not using all of the available spell effects. The combination of a Cloak spell (to kill melee enemies over time) and the stun-locking of the Impact perk make Destruction quite viable.

Obviously, the higher difficulty one plays at, the more one must play a "hybrid" character, using multiple skills. That's true for any "archetype" of character.