Grand plans pushed by French President Emmanuel Macron to reboot the European Union after Britain leaves in 2019 will collide with issues of cold hard cash and sovereignty at the summit in Brussels.
The 27 leaders won't take any formal decisions on Friday but will lay the ground for what could be bitter negotiations ahead, particularly over a post-Brexit funding gap of up to 15 billion euros a year.
They are also set to lay down the law to the European Parliament, insisting that as elected national leaders they, and not MEPs, must have the final say on who is the next commission president in 2019.
EU President Donald Tusk said in his invitation letter to the leaders that he would seek their "guidance" on the issues that will shape the years to come for the continent.
Juncker, who is due to stand down next year, said on the eve of the summit that Europe's recovery after years of crisis was "back on track", but that "we cannot slow down, Europe must press the accelerator."
"We must take it step by step starting tomorrow," he said.
'Power grab'
British Prime Minister Theresa May will be excluded from the meeting in Brussels, the latest in a series of special EU summits aimed at charting a roadmap following the shock of Britain's June 2016 vote to quit the bloc.
One key issue is how the EU will choose a new head of the European Commission, the executive arm of the union, which has a monopoly on proposing new laws and has become increasingly political under Juncker.
In 2014 Juncker was chosen by the so-called "Spitzenkandidat" -- German for "lead candidate" -- system.
That involves the largest party in the European Parliament nominating its candidate for the commission post, after which the European Council of EU leaders makes a final choice taking the pick of MEPs into account.
The European Parliament wants the same to happen after the next European elections in 2019, but with a guarantee that its candidate will be chosen.
On Spitzenkandidat, Tusk will ask whether the EU should accept that the parliament's candidate is "automatically" going to become commission chief or whether the final say must be with leaders.
Officials said leaders were mostly opposed to what they see as a power grab by MEPs at the expense of national sovereignty.
"There cannot be, and will not be, a guarantee that one of the lead candidates will be president of the commission," a senior EU official said, adding however that leaders wanted to avoid any "deep conflict" with the parliament.
'Unrealistic' budget timeline
The other thorny issue is how to fill the hole in the EU's multi-year budget after 2020 left by Brexit.
Tusk is going to ask leaders whether they are ready to pay more and to urge them to focus on defence, education and migration. Brussels has suggested cuts to agricultural funds and payments to poorer regions.
He will also say it is "unrealistic" to expect any agreement on the budget before the 2019 European elections, pointing out that last time it took 29 months to reach agreement.
Tusk will also tell leaders that he expects to go ahead as planned at a summit in March and issue Brexit negotiating guidelines on the future EU-Britain relationship, whether or not the British premier has offered her own vision, officials said.
He will also push ahead with that even if there is no deal on a post-Brexit transition period, currently mired in negotiations about how long it will last, how closely Britain must stick to EU laws, and the rights of European citizens.
May is expected to make a speech next week on Britain's partnership with the EU after Brexit, her office said Thursday, after she chaired an eight-hour cabinet meeting at her country retreat to thrash out a plan.
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