I vote NONE - leave the TSP as is. Sadly many of the federal employees I work with have little understanding of mutual funds or their TSP choices, so the fewer choices for them to flounder with, the better. Recently the TSP started enrolling new employees automatically in an age-appropriate Target date fund (L funds) rather than the G fund, which I think is a good move. TSP has a small range of diverse funds, not comprehensive, but index funds, and they are dirt cheap. Even Allan Roth said he wants to be a fed so he can invest in TSP - he mentioned this at Bogleheads 11!
I suspect it is the mutual fund industry who want to get their fingers in the TSP pie and offer REITs and other options, but we can get those separately in our Roth IRAs, at leas those of us who understand asset diversification and what REITs are