So, season 5 will be coming out in '15 and got to wondering who would be the arc villain(s) for this season. Personally, I'd like to see Grogar. Any thoughts on these lines?

3750453 I say we revivie another gen 1 villian.
That ooze thingy!!

3750472 That would be the Smooze. I could see that. Or maybe both of them! After all Tirek teamed up with Discord, so it's not out of the realm of possibility!

3750524 Chrysalis, okay; the sirens, I'm hesitant towards. (Namely on the basis that their necklace jewels were shattered in EG world, so... what would that do to them in Equestria?)
And I have to agree with you on Fluttershy of the Mane 6. (Although I agree on the basis that she is a woobie moe character.) Personally, my favorite pony is a tossup between Big Mac or Sweetie Belle.
On a separate note, where do people find these moving pictures?

3750453 My guess is they invent a new villain that Celestia forgot to mention (Again). Preferably though, I'd love to see a rebooted villain from a previous generation, like Tirek

3750693 My memory ain't what it used to be. Who did she forget to mention the first time?

3750703 Sombra and Tirek. Through the show, we know about Nightmare Moon and Discord, but at season three and four, they appear in the we-thought-they'd-never-come-back fashion, and Celestia doesn't even mention them until it's convenient. Plus, every time I think I have the "Before Nightmare Moon" timeline figured out, a new villain from the past pops up

3750734 Well to be fair, given Twilight's personality she wouldn't be satisfied until she accounted for every possible threat through her OCD methods (need I mention the incident occurring in 'It's about Time' and in it, there wasn't anything to worry about in the first place). I will admit, Celestia probably leaves out pieces through omission, but given what generally happens in a panic (most of the ponies running around with their heads cut off) can you blame her?
As far as the 'Before Nightmare Moon timeline' (which is aptly named, I thought) there are probably more theories than you can shake a stick at. And there's one error that's 'In-Canon.' (Involving Sombra strangely enough.) However, I have theories on that one (and it's a doozy too!)