Wdyt of using the nickname [name]Wren[/name] for [name]Serena[/name]?
I think it could work. It may not be something that people naturally gravitate towards, but if you call her [name]Wren[/name], everyone else will catch on.
I feel like I’m pretty liberal with nicknames, and I confess I don’t hear or “feel” it enough. I agree that if you call a girl [name]Wren[/name] all the time, people will call her that, and there are certainly people out there with nicknames, like [name]Bear[/name], that don’t go at all with their given names, like [name]Leanne[/name]. But I wouldn’t personally link [name]Wren[/name] with [name]Serena[/name]… [name]Sera[/name], [name]Rena[/name], Sea (prn like see-uh or see, either way) all feel more natural.
It’s too much of a stretch. Firstly, there’s no ‘w’ (bug bear of mine: [name]Lauren[/name] and [name]Maren[/name] etc should be [name]Ren[/name] not [name]Wren[/name], what’s the point in adding a silent letter? Natalies are never Gnats or Knats :p) and secondly there’s no ‘ren’ sound. [name]Serena[/name] is ‘reen’.
The Welsh name [name]Seren[/name] would work though! [name]Seren[/name] nicknamed [name]Ren[/name].
I love both [name]Wren[/name] and [name]Serena[/name]! Honestly, if you love it, I think it could work. A couple years ago I tried to do [name]Serena[/name] nn [name]Wren[/name], but I got lots and lots of comments about how it was a huge stretch. But if [name]Evelyn[/name] (EV-eh-lin) can get [name]Evie[/name] (EE-vee), then why can’t [name]Serena[/name] get [name]Wren[/name]? As for [name]Wren[/name] vs. [name]Ren[/name], I think you could easily opt for the “W” spelling. The association to the bird is much better than the association to a gnat. 
Plus, [name]Ren[/name] has always looked harsh and quite masculine to me (maybe thanks to [name]Ren[/name] and Stimpy? I don’t even know what the show is, lol, but I hear it brought up all the time when [name]Ren[/name]/[name]Wren[/name] is, and I know [name]Ren[/name] was a guy…). I think [name]Serena[/name] nn [name]Wren[/name] is lovely, albeit a bit of a stretch. The negative reactions put me off of it, but maybe you won’t get the same response. I think you could even do [name]Wren[/name] [name]Serena[/name]–the sound is different enough.
I agree with pps. It’s too much of a stretch and there aren’t any sounds that would suggest the nickname, even at a stretch. I think [name]Serenity[/name] nn [name]Wren[/name] works as a nice alternative.
The difference with [name]Evelyn[/name] nn [name]Evie[/name] is that [name]Evelyn[/name] contains [name]Eve[/name] and I think it might’ve originally been pronounced [name]EVE[/name]-lynn??
[name]Wren[/name]/[name]Ren[/name] as a nickname for [name]Serena[/name] is a bit of a stretch. The letters are there but the sound is off.
It’s too much of a stretch for me, if she came up and said her name was “[name]Wren[/name]” well I’d assume that’s her name, full name, if she mentioned it’s a nickname, my first guess would be [name]Winifred[/name].
[name]Rena[/name]/[name]Rina[/name] would be the nickname I’d pick for [name]Serena[/name].
I think it would be kind of hard to stick to it since it’s not an obvious or common nickname for [name]Serena[/name].