In the 24th century, the planet's fossil fuels have been completely consumed, forcing humanity to use solar power as the primary source of energy. A huge solar energy system orbiting the planet was eventually built, anchored to the Earth and providing it with power through three orbital space elevators. As a result of the expense of such a large-scale project, most the world consolidated into three factions: the Union, the Human Reform League, and the AEU. Despite their common reliance on the fragile orbital elevators for energy, however, humanity still continues to constantly fight wars and build up arms. A paramilitary organization called Celestial Being, equipped with incredibly powerful mobile suits called Gundams, fight in order to force humanity to unite and eradicate armed conflict.
Half a year has passed since Sagara Sousuke took up the job of protecting Kaname Chidori and has started living like a normal teenager for the first time. A former child soldier, his view on the perils of everyday life is highly unconventional and in frequent need of violent adjustment; Chidori can only hope her endless assault will eventually get him used to normal life. But a shadow looms over their happy days as Sousuke's employer, the private army called Mithril, is attacked by a new unknown enemy. Suddenly outgunned, can Mithril afford to keep one of their best playing around?
While both Gundam 00 and FMP do have mecha battles, they are even more similar in the main characters. Both Sousuke and Setsuna grew up in the midst of fighting and war. This gives each of their characters a unique outlook to the world. So, I think with a mix of action, militaristic combat, and similar main characters, you would enjoy one if you liked the other. Be aware that FMP has comedy/romance while Gundam 00 does not.
Kajika is the fourteen-year-old heiress of the Burnsworth Corporation who just wants to live a normal life, but her father has other plans. He informs Kajika that her fate is a heavy one to carry and he will only tell her the full details when the man she intends to marry is by her side. As such he has set up a ‘marriage game' for her; Kajika will meet three men that he has specifically picked out and she is to select one of the three to become her husband. There's just one catch; Kajika must also make her chosen man fall for her, since the ‘suitors' are unaware of the game in which they are participating. For a girl who has never loved anyone except for her pet leopard, this game will prove to be a trial. With her over-protective childhood friend and personal shadow, Li Ren, keeping a close watch, will Kajika be able to choose who will become her lifelong partner and finally learn of her true fate?
Gundam 00 is a shonen mecha anime set in the future with giant robots and advanced solar technology. HanaSei is a shoujo about a game of arranged marriage for a young heiress.
On the surface level, these are two very different shows. However, the political intrigue present in Gundam 00 is one of the reasons why I view it in higher esteem than other Gundam series. In the same regard, Hanasakeru Seishounen is much more of a political mystery about power play in a volatile Middle Eastern country than a romance story.
Gundam 00's arc about Azadistan is very similar to Hanasakeru's plot. Both series deal with a fictional Middle Eastern country dealing with impending civil war caused by an energy crisis. The protatgonists of both series are idealists from outside factions who have a hidden relation to the country that's facing conflict. Coup d'etat, religious extremists, foreign interference, a tense political atmosphere, multinational political gambits - common elements of the two series.
Gundam 00's plot has a broader scope involving many more conflicts in other locales than just the Azadistan crisis while HanaSei focuses on one conflict. Hanasakeru Seishounen's story starts out a bit vapid with lots of fluff and humour but turns out more serious as the political intrigue unfolds. If you've watched Gundam 00 and want to see the Azadistan arc drawn out to a full series, wish there was romance and loved the political side of the plot, you might want to try Hanasakeru Seishounen. If you've watched HanaSei and don't mind forgoing the romance for more politics, watch Gundam 00 for a dose of that and some action-packed mecha battles.