1) He may have to temporarily step down as Deputy First Minister. (while any campaigning goes on)
2) If he wins he will have to step down and be replaced - who will replace him?
3) If he wins he will also have to meet other heads of state - including Queen Elizabeth II. Remember Sinn Fein refused to meet the Queen when she visited Ireland (apart from one SF member in Cashel)
4) It would also mark a huge move forward for the party. Remember it took them a few years to gain a mark of 'legitimacy' in Northern Ireland amongst Unionists and Nationalists. In the Republic Sinn Fein are still a small party. Voters in the Republic still view them as a 'Northern and tainted party' out of touch with mainstream Irish politics. They have a small and devoted working class and border vote. This may be changed fundamentally if Martin McGuinness gets elected.
5) This will also enhance SFs claim to be an all Ireland Republican party - battling to win a united Ireland by peaceful democratic means.
Watch this space!!!