In deregulated market environments, transmission system expansion planning becomes a troublesome task as transmission system operators lose their direct control over plant investments. Moreover, the increase in the share of renewable sources in electricity generation creates an uncertainty about future generation investments in terms of location and size. Assessment of transmission connection capacities, which determines the boundaries of new generation amount that can safely be connected from a region, is a mechanism that facilitates economic and reliable transmission expansion. In this study, use of two novel indices, namely region-based evaluation index (REI) and line-based evaluation index (LEI), is proposed for this purpose. REI aims to indicate the suitability of a region for new generation investments from a holistic point of view. Its calculation is based on the loadings of the lines under N-1 conditions, and their overloading probabilities. LEI, on the other hand, is usable to assess each variation of the considered line's loading. By tracking LEI of a line, one can follow if a line's loading exceeds limits during contingencies, and observe if the loading benefits from additional generation. It allows identifying the weak points of the analysed system. The proposed methodology was demonstrated on projected Turkish Transmission Network model of 2026.