It could also be a Christian Polity. Turkish domination of southeastern Europe was far from inevitable even in the 15th century
I doubt it. The Roman realm was plagued by structural weaknesses which made it prone to internal struggles, civil wars even. Bulgaria was also a constant threat, while the question of Latin/Frank duchies in Greece would be a constant source of tension with Western realms.
The Latin/Frank duchies couldn't build a big, strong and stable empire which could withstand the onslaught of hostile forces from all sides (and from within). The case of the Latin Empire demonstrated that well.
Serbia could never hope to build a lasting empire either. The Empire's attention would inevitably be split between Hungary and whatever else remains outside of its range on the fringes. Its core area would also be too far to keep Thrace secure.
Bulgaria might be your best bet. Although the 2nd Empire did end up fragmented, if the Crusade ends up reviving the unified state, it might be able to keep itself together.
All in all however, the best way to keep the Turks out of the Balkans is probably via giving Gallipoli to Venice and by Bulgaria being dominant inland. Venice is wealthy and powerful enough to maintain a strong garrison and keep the defences intact and up to date, while the prospect of overseeing the trade through the Dardanelles would be a good enough incentive to keep the Venetians committed. Genoa is a good alternative.