In my personal opinion i don't think that he should have a harem as it would ruin if for me because i believe one man one woman, and i think that woman in Issei's case has to be Rias.
I think that also it should turn out with the following couples
- Issei - Rias
Akeno - Vali Yuuto - Xenovia Asia - Genshirou Koneko - Gasper (i know Gasper is a cross dresser but they would look good together). Rossweisse - Odin (it's the only one i could think of) As for Irina can stay for ever alone so she does not fall also ravel phenex can just go back to her clan.
Of course for this to happen there has to be something like due to Issei's feelings to rias his dragon blood no longer attracts other women and then him and rais get married and have a kid who they call Lala (
jk but i did like that anime it was a shame for to love ru to end with that ending).
Another thing is if Issei gets married can he still have a harem? Because if not then it would be beneficial for Rias and Issei's relationship, i mean i am pretty sure that if another girl even has sex with him she would go on a rampage killing the girl who had sex with him and then punish Issei. Please reply thanks.