Aside from Clegg and possibly Norman Lamb, I don't think there are any high-profile Lib Dem MPs left.
I'm not sure who could take over from the current crop of MPs. All of them have at least had some political experience:
Alistair Carmichael, Orkney & Shetland, has government experience as Scottish Minister, but I'm not convinced that he's the best choice to speak nationally. Orcadians and Shetlanders, as I've said before, are still more likely to support that candidate rather than the party and may resist their MP being occupied with national issues. Furthermore, the unique demands of that constituency (requiring to serve to groups of islands that are notoriously difficult to travel between) could mean that he would be unable to assume the role whilst serving his constituents.
Tim Farron, Westmorland & Lonsdale, was president of the party, has held his seat for 10 years to date, and served as environment spokesman for a while. Maybe not the most impressive credentials, but you've got so few MPs to choose from. He could go for it.
Greg Mulholland, Leeds North West, has also held his seat for 10 years, and served as health and education spokesman and has some experience as a spokesman and on committees, but not much in a leadership role.
Nick Clegg, Sheffield Hallam, is the likely outgoing incumbent. He won't stay.
Norman Lamb, Norfolk North, is probably the most likely candidate. He has slightly more experience (elected in 2001), has ministerial experience in the coalition, and I suspect is the most likely MP to take the role.
Mark Williams, Ceredigion, hasn't really been a high profile Lib Dem, and I doubt he really has the credentials to compete against the other MPs. On the other hand, given the dire result, perhaps a fresh face is what's needed?
Tom Brake, Carshalton & Wallington, has had front bench experience since his election in 1997, but has been quieter as of late. He didn't hold any particularly important or high-profile offices in the coalition. Possibly a credible candidate, but I suspect he's not aiming towards leadership.
John Pugh, Southport, has again had front bench experience, but has focussed on back bench activities during the coalition. Again, I don't think leadership is his aim.
I'd agree that none of them are likely to be credible candidates. However, as has been said, you don't need to be an MP. However, I think you'd have to hold a reasonable office. Perhaps the Lib Dems as a pro-European party will choose one of their 12 MEPs, who might be in a good place to support our membership in the EU in our forthcoming referendum.
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Leader of a Party doesn't have to be a MP - see the Greens, UKIP or SNP. You could in theory put Vince Cable as leader despite him losing his seat but as someone who's unlikely to stand in 5 years time that probably wouldn't be a good decision.
True, however SNP's leader is the First Minister of Scotland, and UKIP's leader is a high profile MEP. Only the Greens are represented by a politician who does not currently hold office.
I think it's unlikely that a party would nominate someone who has just suffered a crushing defeat to lead it going forward!