After years of experience, handling the Divine's spy network and keeping her safe from assassins, I'd say Leliana is capable of protecting herself. Her open-door policy regarding Chantry rules is commendable but it's the aftermath of her ascension to the Sunburst Throne that worries me. In the unhardened version of her rule, she only manage to keep the peace for the time being whereas in her hardened version of her rule, she turns to bloodshed. Either way, the next Divine after her reign is going to have her hands full. If she fails to live up to Leliana's example (reverting back to useless tradition or getting herself assassinated), then the bloodshed that occurred during Leliana's reign was all for naught.
If Leliana becomes the next Divine, I keep thinking that Leliana's relationship with the Warden will be strained. The Imperial Court (or wherever) will use their relationship as a weapon against them. Look what happened to Briala and Celene. I know Leliana will change the rules about the Divine's personal affairs and such later on but imagine the sh*tstorm they'd have to go through. More than a decade's passed and they were separated with a continent between them most of the time, it's a miracle their relationship can withstand it all.
Also, consider the fact that it was the Grey Wardens that killed Divine Justinia. I don't know about the Warden's outcome about searching for the Calling cure but I doubt they will rid themselves of the taint until they feel the Calling is truly upon them which won't happen in another decade or less. Till then, the Warden's presence near the Divine is one helluva threat. Let's also not forget that the Warden has better things to do than standing around next to the Divine looking pretty. With all that's happened to the Grey Wardens, I'm sure the Warden will be called upon to help rebuild the Order in southern Thedas.
Again, these two love birds will be separated. I can't imagine anyone who's been away from each other for so long can still maintain a positive relationship.
It's up to the players' choice really. Are they willing to put duty over personal affairs? Perhaps Wynne was right all along. Love is ultimately selfish. Even the most carefree/forgiving/accommodating Warden will feel caged. No adventuring, no threesome/foursome with some random lady pirates gallivanting, they have to behave themselves bla bla bla... Years of serving Divine Justinia did not do Leliana any good despite them being close to each other. Who's to say that won't happen for the Warden who remains with Leliana if she becomes the next Divine?
That being said, I recall Leliana saying "There's no happily ever after, not when life goes on." Tru dat. *shudders*
I don't know about you guys but I'm sticking with an unhardened Leliana who isn't next in line for the Sunburst Throne, waiting to return to her Warden after the Inquisition's done its work.