The game gets "very boring" to you. Do you understand that just because you find it so, that doesn't mean it's true for anyone else? You are speaking for yourself; this is your personal opinion, not a "fact".
You do not "finish" this game. There is no "ending". You did not "complete" this single player game. You, personally feel you've finished it. Good for you. That's not a universal opinion and it's not how the game is even designed. The story does not 'end' after you finish any quest at all. And to you, personally, you have "nothing to show or do".
And lastly, it is absolutely not true in the slightest that "nothing has changed" in this series of games other than a new storyline. I really don't think your opinions carry much weight when you confuse your playstyle with the way things should be, and you cannot see the differences in each and every installment of the ES series
The games finished when you end up doing repetitive things. Im not the only one who says its boring i have friends who have played the game enough to realize nothing new is haopening all thats going on is: get quest, go duengon, kill boss or target, back to quest giver, get petty reward and repeat.
Ive also seen way too many posts that talk about bordem because those ppl have finished the main story line and the sub story lines that arent repeats of the same stuff over and over.
As for what could be changed in future games to impliment the element of multiplayer; quests that have to end up by killing a certian npc have to be changed so that after the npc is disposed of the player who finished the quest cannot the that certian noc and only others can see it.
Also think of the things you could do if multiayer was in a future TES game or this in the future. You could make your own type of guild take over territories, hunt others down, control citys or forts and stuff against others, and much more.
A bounty system could be implimented so you can bounty others to be killed by certian ppl. The possiblites are endless in a great open world like this.