One final warning, this has potential spoilers!! It could easily spoil the rest of season 6 and some of season 7 for you!!
LIKE HUGE SPOILERS GUYS
Ok, enough with the warnings. Here's my theory...
Glen isn't dead.
I know, you've heard it before. "The zombies were eating the guy on top of him, there's enough room under the dumpster for him to slide under." Even better than this, though, is that (to quote the Walking Dead's wiki) "ACTOR JEFFREY DEAN MORGAN HAS OFFICIALLY BEEN CASTED AS WALKING DEAD COMIC VILLAIN NEGAN!"
What does that mean? Who is Negan? Well, we have to go into the comics to find that out. Basically, Negan is the next governor. He's the next 'big boss' that the group will have to fight, at least according to the comics. Let's look again to the Walking Dead's wiki: "In the comics, Negan is the notorious foul-mouthed leader of a group called the Saviors."
The Saviors... Could that be the group that the group of three Daryl stumbled upon in Episode 6 were running from? There was a lot of talk of 'not going back' and 'not spending anymore time on our knees', implying that the leader of this new group relies on fear to scare his group into following him. Again, from the Walking Dead's wiki... "[The Saviors] use fear to rule over Negan’s community and enslave women to be his many “wives.” The Saviors were ordered to attack neighboring communities if an ongoing ransom wasn’t paid and savagely beat *SPOILER* to death in front of the surviving members of Rick Grime’s unwavering group."
Definitely fits the descr
iption, in my opinion. Now, how does that relate to Glenn? Note, from the last quote, this portion of the final sentence: "...and savagely beat *SPOILER* to death in front of the surviving members of Rick Grime’s unwavering group."
Who is that spoiler? If you guessed Glenn, you're right!
In the comics, the group is ambushed by the Saviors at night, and Negan randomly chooses Glenn to kill (beat to death with his wire-wrapped bat named Lucille) in front of the group. This leads to Maggie leaving the group, and raising their child (currently unborn) on the hilltop where Glenn is buried.
Other theories suggest that they may end up having Negan kill off Maggie instead, or Daryl/Morgan. What is pretty clear, though, is that much of season 7 will be spent in a war with the Saviors.
(Sources:
Walking Dead.wikia.com,
Wikipedia's Page on Negan - Sources may contain foul language)