I think that giving that Mercedes seat to Alonso wouldn't be as bad as a lot of people say but it wouldn't be in my opinion the best choice out there. I think that McLaren are lucky to have Alonso or things could've been worse for them. Maybe Alonso and Hamilton changed and are more mature, but that's a risk I wouldn't take.
A lot of people are mentioning Wehrlein; I personally don't think it's a good choice either. Force India turned him down to get Ocon instead. Let's not forget Wehrlein didn't dispute the whole F1 season and was driving a Manor. Yes, he has talent but no, he's not ready to get that Mercedes seat. At least, Mercedes shouldn't rush him. I think Mercedes wants to see Wehrlein compete in a better car before giving him the Mercedes seat, hence why they tried to put him in Force India. Wehrlein might stay in Manor or join Sauber this season.
Now in my opinion, the most probable thing to happen is that we see Button take that Mercedes seat. Yes, he officially retired but he also admitted he thought he retired too early without clearly saying if he regretted the decision or not. The wise choice would be to get him for a year then let him retire for good. With the surprise Rosberg retirement, I think Button could consider driving the best car of the last three years. First, Hamilton and Button have history together and secondly, Button could be a safe choice to be the clear number two in the team and earn important points for the constructors championship. He could also be a good sponsorship asset and would cost less than it would to bring Alonso. Then maybe, we could see either Ocon or Wehrlein, depending on their season, get that Mercedes seat.