Think about what SG want from us before asking the players about what they want from SG .

We are still waiting for the Grace of Unity feature

Your example is a primary reason they are unlikely to allow this in any meaningful way. While I'm sure you only mentioned Vivian/SSV as an example b/c SSV just came out, this is precisely what I expect they reasonably want to avoid being widespread - people piling into the newest/latest ML. At a minimum they'd want to avoid such a liberal system of shards, so even if going with some sort of system like the one you and many others previously have suggested, they would likely attach further stipulations such as imprint copies only being useable on like original element; so for ex., any Vivian dupes would only create imprint mats for 5* Earth units. 

In general, I think it's a headache SG is hoping to avoid dealing with. Any system they come up with people will endlessly complain about not being open enough, so will just add something else they constantly have to tinker with. IMO it's not a feature necessary to this game. 

But as meta comes and goes, certain characters would become obsolete

==> same for most, if not all gacha games.

I'm not suggesting 1:1 exchange ticket to another character with the same rarity everytime we recall someone,

==> Not happening ever even if not same rarity/material etc. This nets them no profit as all gacha companies want you to buy and buy even more.

This would help new players decide which character worth investing while they are still learning the game without affecting their progress

==> New players should research before investing? Isn't this what most people do when they start out gacha game? like looking for which character is best to reroll, invest etc?

I can see third point being a thing but I still don't think they will not give us this option except the only one they mentioned previously in awaken update something about temporarily maxing a character if you have 60 or something, kind of like trial period to test a character with full mola invested

"for example, you have SSS Vivian for Banshee 13 when you were still a new player.."

Are you off your rocker dude? What new player has sss anything and why would a new player not only invest in banshee first but also try to make a one shot comp with Viv. Your example isn't an example because it's never happened in the history of the game.

The grace of unity feature allows anyone to test or even use any hero at any time and eliminates basically your entire post. As far as the bits about having a hero you don't use anymore, there have been many times where a hero gets no changes at all and just comes back because either the PvE content or PvP meta demands it. Just because you don't use something you have +15d now doesn't mean you won't use them in the future regardless of buffs

Nothing you want will ever happen because it allows all players to just recycle their stuff which kills a massive part of any farming/gacha game's longevity and revenue stream; time-gated materials.

Devs have announced about this future feature.

Shout out to epic7#u1vbei for finding the source.