As far as I know, all we have on the question of Lenin's plans for the future of the USSR are some letters that he wanted to be read to congress, but which were suppressed by Stalin after his death. In these letters he seems to suggest that there are three major candidates for the leadership; Stalin, Trotsky and Bukharin. He ominously warns for Stalin's position within the party:

'Comrade Stalin, having become Secretary-General, has unlimited authority concentrated in his hands, and I am not sure whether he will always be capable of using that authority with sufficient caution.'

In an addition to the first letter he also notes:

'Stalin is too rude and this defect, although quite tolerable in our midst and in dealing among us Communists, becomes intolerable in a Secretary-General. That is why I suggest that the comrades think about a way of removing Stalin from that post and appointing another man in his stead who in all other respects differs from Comrade Stalin in having only one advantage, namely, that of being more tolerant, more loyal, more polite and more considerate to the comrades, less capricious, etc.'

It is easy to read into these passages some kind of prophetic warning of a ruthless one-man dictatorship initiated by Stalin, but the reality is quite different. In order to understand Lenin's 'testament' as these letters are referred to, we also need to understand some historical context. The position of general-secretary is at this point not synonymous with the position of party leader. The general-secretary was the person on the politburo that was responsible for the (composition of the) membership. This meant that he had the power to expel people from the party and also oversaw new recruitment to the party. In light of this, it makes sense that Lenin would want to see Stalin replaced because he perceived him as intolerant, rude, inconsiderate etc. which are not especially great characteristics to have for someone who is supposed to notify people that they are no longer part of the party or, more importantly, attract them to it. For the tasks of a general secretary it would be rather more convenient to have someone who is friendly, cheerful and generally charismatic.

The remark about Stalin's inability to employ his powers with caution is not a warning for a coup but a warning of escalation of tensions between the supporters of Trotsky and Stalin, possibly leading to a split. He mentions his fear of a split a couple of times in the letters and it seems to be the red line (pun not intended) in his criticism of the aspiring party leaders. Although his assessment of Trotsky is a great deal more positive than that of Stalin:

'Comrade Trotsky, on the other hand, as his struggle against the C.C. on the question of the People's Commissariat of Communications has already proved, is distinguished not only by outstanding ability. He is personally perhaps the most capable man in the present C.C., but he has displayed excessive self-assurance and shown excessive preoccupation with the purely administrative side of the work.'

Most likely his preferred candidate was Trotsky, but he also realized that outright support would very likely do more harm than good to the party in that it would only speed up the process of a split between supporters of Stalin and Trotsky.

He notes Bukharin's qualities and his popularity as well as a short appraisal and criticism of Pyatakov, but does not go any further into the matter.

In order to deal with the instability in the party, Lenin proposed to greatly expand the amount of seats on the central committee and assign most seats to workers and peasants that had been loyal party members. According to him, this would secure the future of the Soviet system and place the renewal of the administration under the supervision of (representatives of) the working class.

In conclusion, we might suspect that Lenin preferred Trotsky as his successor but he did not voice this explicitly as he was more concerned with a possible split within the party. To prevent this split, he proposed a reorganization of the Central Committee. Inevitably, the choice of Lenin’s successor was not up to Lenin but to the Party.

Thanks!