Please explore the legal feasibility of having an Annex to a Treaty/Protocol (which as such would have to be ratified/enter into force) in which Parties would inscribe targets and which would automatically become legally binding, i.e. without having to go through some additional amendment ratification process (as in the case of the Kyoto Protocol). In other words, the declaration by a Party that they wish to have a certain target recognised as internationally legally binding should be sufficient for it to be so (maybe with the precondition of a COP decision expressing consent).
From a legal perspective, it is perfectly possible for treaties to contain general rights and obligations, which can be rendered more specific/concrete pursuant to subsequent decisions of a ‘Conference of the Parties’. In the context of international environmental law (and elsewhere), such practice is rather rare, since States Parties usually wish to retain ultimate control over the specific obligations resting upon them, rather than to have a COP decide by (qualified) majority on the obligations to be imposed. One notable exception, for instance, is the Montreal Protocol on Substances that Deplete the Ozone Layer. Article 2(9) of the latter Convention provides that the COP can adjust the ozone depleting potentials and the reductions of production or consumption of the controlled substances, thus amending (read: strengthening) the obligations imposed on the various States Parties. The provision states that the COP should strive to reach agreement by consensus, but can, as a last resort, take decisions binding on all Parties by a two-thirds majority vote of the Parties present and voting.
In a similar vein, it is perfectly possible for a treaty to lay the groundwork for obligations which are subsequently specified by means of a roster where Parties inscribe concrete targets. This follows, on the one hand, from the fundamental principle that treaties are binding on the Parties and must be implemented in good faith (‘pacta sunt servanda’) as well as, on the other hand, from the principle that unilateral statements and declarations can create binding obligations if (1) they are public or generally known, and (2) reflect the intention of the State to be bound. The International Court of Justice has indeed made clear that unilateral statements and declarations constitute a source of international law subject to certain preconditions. The International Law Commission has clarified those conditions in its 2006 set of Guiding Principles.
In light of the foregoing, nothing prevents States from concluding a framework treaty, which foresees that concrete obligations of individual States are specified in a separate annex, and which moreover provides that the obligations can be amended (strengthened) over time (without the need to go through a cumbersome amendment procedure for the treaty itself…). It would, for instance, be possible to stipulate that States can unilaterally assume higher targets, but cannot unilaterally lower their obligations. Amendments of the individual State commitments could be linked to a prior decision of the COP, but this is not necessarily so – everything depends on the wording eventually chosen.
In the margin, it is worth recalling the 2009 Copenhagen Accord. The latter ‘Accord’ indeed foresaw that States would unilaterally communicate individual commitments to be included in one of two Appendixes added to the Accord. Nonetheless, both the text (and context) of the Copenhagen Accord, as well as the wording of most of the individual commitments (framed in conditional terms or explicitly stressing the ‘non-legally binding nature’), made clear that the various commitments were of a political nature, and were not be regarded as legally binding. By contrast, for the proposal in the query to be effective, it would be imperative that the legally binding nature of the commitments is clearly stipulated.
 See http://ozone.unep.org/pdfs/Montreal-Protocol2000.pdf. Note: reference can also be made to the International Convention for the Regulation of Whaling and the Schedule attached to it, for instance: the International Whaling Commission can indeed make decisions amending the Schedule (thus deciding on which whale species can be hunted and under what circumstances). An important difference with the Montreal Protocol is that States Parties will not be bound by the decisions of the International Whaling Commission if they submit a formal objection to this end.
 See in particular the Nuclear Tests judgment of 1974. See http://www.icj-cij.org/docket/?sum=317&code=nzf&p1=3&p2=3&case=59&k=6b&p3=5.
 For further information, see ecbi paper by B. Müller, W. Geldhof and T. Ruys, ‘Unilateral declarations: the missing legal link in the Bali action plan’, Mary 2010, 29 p.