When we finally get Balance Breaker, Glorious Solar Plate would almost be redundant, and a waste of XP, since we'll be able to summon Artifact armor at will.
What Abilities to raise?
Hmm...
Kind of depend on what direction we want to go in.
A couple of dots in Socialize would be good. Help with making allies, and juggling them, like our connections with the Fallen Angels and Devils, and probably Heaven eventually. Plus gathering resources
We'll likely want Investigation maxed out when we get to some of the later story arcs.
Five dots in Craft would be good. That way, we can make a ton of money by selling artwork of various kinds. Especially if we get a specialty like Beautiful Women, which means that, as long as what we make involves the image of a beautiful woman (or is meant for one) we got it in the bag.
War would be good, as we could then train Rias into a great general, to help with her Peerage.
Maxing out Occult and Lore would also be good, as Occult would probably help us with new magics, and we could use Lore to get a job in computers, or
Bureaucracy would also help. For one, it would be a good thing for us to train Rias in, as she's going to (eventually) be ruler of a significant area of the Underworld. Plus, as we're likely going to be her husband, it'd be good for us to know a thing about dealing with governments. Plus, it would let us make more money by way of investments.
Performance... well, we could do a less humiliating version of Oppai Dragon. That show, in canon, made Issei a 'ton' of money. Probably multiple 5 dot Resources.
Mastery of Small Manners would probably be a good thing to get, though I'd like to get Dragon Essence Domination.
If Issei had access to a Sidereal Sifu, I'd suggest he learn a Sidereal MA.
Unless Bright could transfer her knowledge of Sidereal MA somehow (Yeah, didn't think so.)