I would suggest keyword research and planning out how you want them to flow. You will also need to look at your permalinks since you are using wordpress as you might prefer to show the category in the URL

Easiest path would be to just let the post name be the permalink, excluding categories in the url, then you just move them to their respective categories.

Take your time and carefully plan everything out first, you will also need to fix the old URLs to point to the new ones so don't forget that.

Im not sure what you mean about preference. If you mean that if your "geo targeted country" has an effect on improving your chances for terms like "SEO Services in India" then my answer is not so much. You'll just have an improved visibility in the local engines but geo keywords will still take the same amount of work. I've played with this for my site recently. My geo targeting doesnt really affect my global geo related keyword rankings.

Hi

That's a lot of indexed pages. What were the bulk of those pages? Categories? Products? Random tags or thin pages?

Did you switch to https recently? Are all versions included in webmaster tools? With a preferred version? How about their sitemaps?

It's possible that it's panda. It's also possible that it's a technical side like a switch to https, improper redirects, etc.

I just find it weird to see a big site like that lose that much indexed content (assuming they were good pages) without incurring an algorithmic penalty or a technical problem.

Look at Google Trends, Quora, Yahoo Answers, keyword tools that get from Google Instant are also good.

Find forums. Look at Reddit (add /top/?t=all at the end to check out the most popular posts)

Try checking it out with bing/yahoo and yandex. See what type of pages they were then go to archive.org to check them.

If it's indeed thin pages or pages caused by parameters/searches/sorting then that is usually an automatic algorithmic penalty, usually, panda.

You might also have some good pages with good links, so be sure to check for 404s. 301 good pages as needed. It's a really tricky situation.

You also have to see if there's trouble with internal links and inbound links (just in case there's a mix with penguin. I've seen sites with manual and algorithmic penalties for both wtf)

Try out tools like

http://www.barracuda-digital.co.uk/panguin-tool/

http://fruition.net/google-penalty-checker-tool/

to see when your drops really started. You should be able to identify your algo penalty at least and go from there.