Dragon not appearing at Bonestrewn Crest

Dragon souls can be saved, so don't worry that the effort in killing the dragon is lost.

Please note that you need to discover additional shouts if you have learned/activated all your current words, this way you can use your saved dragon souls on the new words of the new shouts.

Shouts are discovered on the walls of old ruins, and gained by slaying and absorbing the souls of dragons. As more are slain the users ability to learn and use new shouts increases.

— Wikia - Elder Scrolls V: Skyrim - Dragon Shouts

In order to learn new shouts, you need to find all these word walls; you can get a quest to the next:

Although finding word walls can be very challenging try and use shouts in major cities. Whenever you do this you should receive a note from a courier which tells you "a note for your hands only" this note starts a quest to find a source of power and the current location of the next word wall.

— Wikia - Elder Scrolls V: Skyrim - Word Wall (Contains more details on all the locations)

To actually learn a new shout, once you have discovered the word wall and have a dragon soul to use, you need to manually activate it in the shouts menu. Select the shout and press the Activate button. It should be shown on your screen, but I believe the buttons are:

• R on PC

• X on Xbox

• □ on PS3

So here's what I know you can do (name followed by the quest you should start but not complete):

People

• Any normal follower (or, hey, Aranea Ienith can summon an atronach)
• Delphine + Esbern - Alduin's Wall (Esbern can summon an atronach)
• Brother Verulus - The Taste of Death
• Mercer Frey - Speaking with Silence
• Thonnir - Laid to Rest
• Avulstein Grey-Mane + Geirlund + Vidrald - Missing in Action (the latter two should join you and Avulstein at Northwatch Keep)

Animals

• Meeko (Meeko's Shack) or Stray Dog (random encounter)
• Barbas - A Daedra's Best Friend
• Horse (better yet, Shadowmere)

Summons

• Two summons (with 100 in Conjuration and the Twin Souls perk). Some dead thralls can summon atronaches, so that gives your summons one summon apiece.

which brings the total to you, nine people, two dogs, a horse, two dead thralls and four atronaches. That's 13 permanent and almost everywhere (excluding atronaches and the horse) and 18 potential. This seems pretty hypothetical and I haven't done it myself, so feel free to correct me.

Other

There's also supposedly a glitch (patch status unknown) where you can dismiss your follower, hire someone else, then immediately return to your follower and rehire him or her. I don't know if people use it with the Dark Brotherhood Initiate(s) for a reason or because it only works with them.

Also Odahviing.