I think it's also important to mention [relativistic effects][1] here.  They already start becoming quite visible after $Z=70$, and Ra lies a good bit after that.

In very heavy atoms, the electrons of the $1s$ orbital (actually, all orbitals with some electron density close to the nucleus, but the $1s$ orbital happens to be the closest and therefore most affected) are subjected to very high effective nuclear charges, compressing the orbitals into a very small region of space. This in turn forces the innermost electrons' momenta to be very high, via the uncertainty principle (or in a classical picture, the electrons need to orbit the nucleus very quickly in order to avoid falling in). The momenta are so high, in fact, that special relativity corrections become appreciable, so that the actual, relativistically corrected momenta, ($p_{relativistic}=\gamma p_{classical}$) are somewhat higher than the approximate classical momenta. Again via the uncertainty principle, this causes a **relativistic contraction** of the $1s$ orbital (and other orbitals with electron density close to the nucleus, especially $ns$ and $np$ orbitals).

The relativistic contraction of the innermost orbitals creates a cascade of electron shielding changes among the rest of the orbitals. The final result is that all $ns$ orbitals are contracted, getting closer to the nucleus and becoming shifted down in energy. This is relevant to the question because the $7s$ valence electrons in Ra are more attracted to the nucleus than one would expect from a simple trend analysis, since they rarely take into account the increase of relativistic effects as one goes down the periodic table. 

Thus, the first (and second) ionization energy of Ra becomes higher than expected, to the point that there's actually a upward blip in the downward trend. Eka-radium ($Z=120$) would have far stronger relativistic effects, and can be expected to have a significantly higher ionization energy compared to Ra. In fact, relativistic effects will conspire to make the group 2 metals slightly more noble! Though the periodic table becomes such a mess near the superheavy elements that it's hard to say whether it'll be a clearly visible trend, or just one effect to be combined with several others.


  [1]: http://www.annualreviews.org/doi/abs/10.1146/annurev-physchem-032511-143755