An analysis of Marriage options for Daenerys, taking into account only the show:
Dorne
No potential spouse character has been shown to exist in Dorne, currently. Even if we ignored the non-existence of a suitable candidate, it is highly likely that Daenerys would want a spouse that has ties to Dorne since it is already supporting her.
The Reach
Randyll Tarly is already married, ser. Dickon Tarly is unwed. However, the Tyrell remnants, and presumably the Reach, are already supporting Daenerys. I could only see this happening in a bid to repair relations if things somehow soured between her and the Reach. Very unlikely.
The Stormlands
No potential spouse character has been shown to exist in the Stormlands. Since the Stormlands does not currently support Daenerys, it is plausible that a Stormlander marriage candidate could show up in the show in order to gain it's support.
The Riverlands
Edmure is already married, which mucks up what would otherwise have been a very plausible and very useful marriage option for Daenerys. The Freys have a paricularly large amount of troops, albeit not quite as many as one of the Great Lords would be able to marshal. However, Freys are hated by many and more. Mayhaps they could contribute a goodly number of troops, but the hatred they would engender to her cause would more than outweigh it. Heh...
The Westerlands
Hypothetically, Tyrion could gain the allegiance of the Westerlands. There are numerous problems with a hypothetical Tyrion marriage. He is despised as a kinslayer. He is already married to Sansa. It is highly dubious as to whether he could ever command the loyalty of the Westerlands. And even if he did, he is already supporting Daenerys which means she doesn't gain anything by marrying him. Extremely unlikely, albeit less unlikely than any alternatives mentioned so far.
The Vale of Arryn.
Robin Arryn is unwed, is the Great Lord of this kingdom, and commands an army that is at full strength. He is probably the second most likely candidate that exists. His familial connections with the Tulleys and the Starks only serve to sweeten the deal.
The North
Targaryens have a long and storied history of intermarriage. As with Robin, this is an excellent prospect as it could serve to win her the alliance of three Great Houses. This is the obvious choice, as has been theorized for years. I'd say the writing is on the wall, at this point.
Darkhorse Option
If (f)Aegon was ever going to be introduced into the show, it might be in order for him to serve as a foil to Jon Snow for the purposes of the marriage plot. Highly unlikely, but certainly possible.