I hear that there is going to be a vote in EASA regarding a 600kg limit in microlights.
You hear wrongly.
The dividing line between European regulation ("EASA rules") and national regulation is set in the Basic Regulation (BR), specifically in Annex II. The entire BR is being recast, in a process that started in 2014, and will probably last until late 2017. The European Commission set out a proposal in late 2015, which made no changes to the current division. Through 2016, the draft was discussed in the Council of the EU (member states) and in the European Parliament. Each of those two institutions has come to a position on where the dividing line should be. Next year, the Commission, the Council and Parliament will try to reach a compromise in a process known as a
Trilogue. EASA is not formally involved.
Whatever happens, the key is in bose-x's question "When does it stop being a microlight......?". Aircraft below 600 kg in the EASA system are called Light Sports Aircraft (LSA). If the weight division were to be raised to 600 kg (or even something lower) it is likely that national regulations for aircraft heavier than current microlights would need to be revised to use the technical requirements for LSA. The way that states choose to implement that is up to them.