I really hope it isn't another MGS game. The fourth installment, though technically proficient, was absolutely boring and ridiculous in term of script, dialogues and cinematics. MGS1 & MGS3 are still, and by far, the best of the series to me and I wouldn't like to see another title putting the now already absurd story any further.
Gameplay was quite good but not outstanding and overall it felt really an uninspired work, compared to its predecesors.
Really, the last two hours of MGS4 were full of badly written plots and kid stuff. Everything Otacon-related was, in Drogba's words, such a ~@#ing disgrace.
And the gameplay was lacking a lot of details and touches that previous games had.
So, I wouldn't like to see a MGS5 because I fear it would turn even worst and I couldn't cope with two of my most beloved series of all times being a flop (PES and MGS).
That said, I think Kojima needs a new franchise to investigate new things. He can be very creative and I feel he's now somewhat constraint by the huge success of MGS and risks going down and loose the touch quickly if he continues to watch his own belly.