Instead of king of Burgundy, Lotharingia, Brabant or Frisia/Friesland could all be alternatives. However there was a kingdom of Burgundy, which was part of the HRE, it was AKA the kingdom of Arles or Arelate, many parts of it ended up in French hands, but the empire had never given up the title. Another thing with the Royal promotion is, what does the emperor gain by granting this crown. Furthermore the other prominent imperial nobles will have an opinion about this too.
One issue regarding the OTL negotiations of Charles the Bold was that he also demanded some other territories not controlled by him (directly) to be attached to his kingdom as vassals.
A kingdom created out of the Burgundian imperial possessions, which still accepts the empire as suzerain is the best bet. Maybe the prince bishoprics of Liege, Cambrai and Utrecht (which were already in the Burgundian sphere) are attached to it and even nominally the territory of the free Frisians; but Cleves, Savoy or Lorraine, which OTL Charles the Bold apparently wished to be given as vassals will be overplaying the 'Burgundian' hand. However there still remains the question, what does the empire and the emperor gain by this.
Regarding the peace of Rouen, maybe Burgundy could keep the counties of St. Pol, Boulogne and in time Calais could be transferred to them. France keeps Ponthieu, Eu, Vermandois, and Picardy.
@ Grey Wolf: Not all kings, since the (Holy Roman) emperor also had the right to grant a royal crown. This Burgundy or Lotharingia could and probably would end up like the OTL kingdom of Bohemia, maybe without an electoral vote (they could eventually end up with one), which was an autonomous part of the empire, which king was an imperial prince and subject of the empire.