Biblical Open theism
According to Watson, "That the sovereignty of God is a Scriptural doctrine no one can deny; but id does not follow that the notions which men please to form of it should be received as Scriptural." Most especially he rejected as unscriptural any doctrine of God's sovereignty that results in making God the author of sin, because this is incompatible with God's goodness. He acknowledged that most Calvinists do not attribute sin to God's causality, but he also argued that their explanation of why Adam fell in the garden necessarily includes or leads to divine causality, even if only indirectly. His own position was that there is no answer to why God allowed the Fall except that God could have prevented it but decided it was better to allow it.  Clearly, for Watson the Fall was not foreordained by God or included in God's antecedent perfect will, but resulted from human self-determination and self-assertion against God, which was allowed by God in his consequent will.
Watson contributed two relatively new ideas to the stream of Arminian theology, although not all Arminians picked them up from him. First, he argued against what he called the "philosophical theory" of free will, which is now generally know as compatibilitst free will. This is the idea of free will advocated and defended by Jonathan Edwards, but its roots can be found al least as far back as Augustine, The idea is that the will is controlled by motives, and motives are provided by something external to the self, such as God. Most Calvinists, when pushed to explain why persons act in certain ways or choose certain things, appeal to the strongest motive as explanation and then add that motives are not self-determined but given to persons by someone or some thing. In this theory people are "free" when they act in accordance with their desires, when they do what they want to do, even if they could not do otherwise. The "free will" is compatible with determinism. Watson rejected it as incompatible with responsibility: "For if the will is thus absolutely dependent upon motives, and the motives arise out of uncontrollable circumstances, for men to praise or blame each other is a manifest absurdity and yet all languages abound in such terms.  According to Watson, the will is not mechanically controlled by motives instilled by something or someone; rather the mind and the will are capable of judging motives and deciding between them. Moral liberty, he argued, consists in thinking, reasoning, choosing and acting based on mental judgement.  Clearly, for Watson, free will means being able to discern and choose between conflicting motives; it includes being able to do other than one wants to do and other than one does. That is the essence of libertarian (incompatabilist) free will. Watson's first contribution was in providing a critique of the Calvinist doctrine of free will (the "philosophical doctrine") and recommending its alternative.
Watson's second contribution to Arminian theology was his denial of God's timelessness, or the "eternal now" theory of God's eternity. He also denied divine immutability. Not all Arminians have agreed with Watson about these matters, but he opened the door to later developments within Arminianism, such as open theism. More importantly, however, he wrestled creatively and constructively with the issue of God's relationship with time in view of the reality of free will and creatures' interactions with God. Until Watson, most Arminian theologian, including Arminius, held to the Augustinian interpretation of God's eternity; that is, there is no duration of God's being in or through time, or there is no real succession of past, present and future in God. Even for Arminius, god's awareness of creation is such that all times are simultaneously before God's eyes. Watson could see no sense in this light of human free will and its ability to affect God's knowledge (i.e., God Foreknows free decisions and actions without causing them). Watson's main concern was to protect free will in order to protect God's character (love) and human responsibility. For him, the doctrines of immutability and eternity as an eternal now were speculative and not biblical. On the basis of biblical narratives illustrating how free and rational creatures affect God, Watson rejected the idea that God cannot change in any way. According to him, God's knowledge of the possible is timeless and not derived from events in the world, while Gods' knowledge of the actual is temporaal and derived from events in the world. Nevertheless, God is sovereign in that he is fully capable of responding appropriately to whatever human beings (or other creatures) do and fitting it in with his overal purpose and plan; God is also sovereign in that whaterver happens is foreknown and permitted by God."
* Note: This is section is from the on-line preview of "Arminian Theology: Myths and Realities" by Roger Olson. His footnote references will be correlated to Waton's "Theological Institutes or, A View of the Evidences, Doctrines, Morals, and Institutions of Christianity" located at the "Wesley Center Online."