Most likely potential candidates to announce yea or nay by the end of this year

[Mr. Morden]

I'll go with Bolton, Bush, and Ryan as the most likely candidates to make some kind of announcement (either saying that they will or that they won't run) by Dec. 31, 2014.  Bolton will be an early announcer if he decides to run.  Bush and Ryan will be early announcers if they don't run.

Reasoning:

Bolton is perhaps the most likely early announcer among those who don't currently hold political office.  That's because he doesn't have private sector ventures that he'd have to unplug from like Carson and Santorum do.  As far as I can tell, he spends most of his time these days with his PAC.  And his PAC is aimed at supporting hawkish Republican candidates in the 2014 midterms.  Once the midterms are over, then he won't have anything to do, so he might as well run for president.   Now, that doesn't mean that he's going to announce in November or December, but he's probably more likely to do so than the rest of the field.

Ryan is aiming for the Ways & Means Committee chairmanship.  He'll most likely get it, but it does mean that the House GOP caucus will have to pick him over someone with more seniority.  They'll probably do it, because Ryan is a star in the party, but Ryan may decide that it helps his case if he reassures his colleagues that he's not going to become a part-time chairman in six months by launching a presidential campaign.  Also, if Ryan isn't going to run, then he might want to give his friend Scott Walker a heads up on that.  (Of course, that's only a consideration if Walker is reelected.)

Bush has already said that he's aiming to make a decision by the end of the year.  And like Ryan, he has a home state ally (in this case Rubio) who would like to know if he's going to run, so he can make his own plans.  Bush isn't as much of a media attention seeker as most of the rest of the field.  He's more of a serious person, and so probably less likely to string out the decision forever just to get more press attention.

[Suburbia]

Bolton- Yes, December 2014
Ryan- No, December 2014 (May reconsider IMO by summer of 2015 if GOP field is uninspiring)
Christie-Undecided
Walker- Undecided (if reelected)
Pence-Undecided
Bush-Undecided
Paul- Yes, December 2014
Huckabee-Yes, December 2014
Jindal-Undecided
Perry-Undecided
Santorum-Undecided
Thune-No (May reconsider IMO by summer 2015 if GOP field is uninspiring)
Kasich-Yes (if reelected)
Peter King-Yes, December 2014
Steve King-No
Cruz-Undecided
Palin-Undecided

Democrats:
Clinton-Undecided
O'Malley-Yes, December 2014
Biden-Undecided
Warren-Undecided
Schweitzer-Yes, December 2014
Feingold-Undecided (He should run for WI-Sen against Johnson)
Bayh-Undecided
Gillibrand-No, December 2014
Kloubchar-No, December 2014
Sanders-Yes, December 2014
Nixon-Undecided (He should run for MO-Sen. against Blunt)

[BaconBacon96]

I think Bolton and Santorum will declare by the end of this year.

[Amenhotep Bakari-Sellers]

Clinton, Paul, and Perry are affirmative.

[Never]

It seems like it would be in Bolton's best interests to announce by the end of this year if he wants to run. That way, he'd have plenty of time to try and lock down the hawkish wing of the GOP.

I could see Perry running as soon as this year. Soon he won't have any responsibilities in elected office, lifting that barrier, and the earlier he runs, the more time he would have to rehabilitate his image. Perry 2016 would have to be different from Perry 2012 to have any serious chance at winning the primary or the general election. On the other hand, he might wait until early 2015 when his term actually runs out before making an announcement.

I'd agree with OP that Ryan would probably say nay to running for president very soon to reassure the caucus. The same might apply to other representatives in leadership positions, like McMorris-Rodgers (though she isn't a very likely candidate to begin with).

[Mr. Morden]

When did we start talking about John Bolton?

June of last year, when Robert Costa reported that he was thinking about running:

https://uselectionatlas.org/FORUM/index.php?topic=164982.msg3772981#msg3772981