I really wanted to dig into the cleric section and get a look at the different domains... and I was disappointed! Seven domains seems far too light for a broad pantheon;
Indeed, way too few... the 3e PHB had 22 domains. The table of deities in the PHB clearly shows that 8 domains are not enough for a good differentiation.
Looking at the list of 3e core domains, I don't think it was a big deal to lose the 4 alignment domains, and to merge Animal and Plant into the Nature domain.
Still others would have been so useful, for instance the elemental domains... While "god of element X" isn't a particularly compelling figure IMHO, the elemental domains would have been very versatile: Fire domain could have covered both deities of destruction and forge (Ephestus/Volcano for instance), Water domain could have covered all deities of the sea like Neptune/Poseidon (Tempest contains more thunder & lightning stuff and is very destruction-oriented), Earth domain would be appropriate for Moradin and other deities related to either mountains, caves, the underground, etc.
And then there are gods of Justice, Luck, non-evil Death, Magic and Travel which aren't well represented with the current 8.
do you think that the DMG might introduce more?
Only the Death domain, and that's strongly evil-oriented, unsuitable for good or neutral deities of the dead such as Kelemvor or Osiris.
Do you have any suggestions for modifying existing Domains for a wider range of options?
The easy part is picking domain spells.
More difficult is to pick channel divinity abilities. You could re-use the existing ones, but you'll be shoehorning them (at least, you can try to keep their mechanic but change their targets).
The most difficult part is to come up with unique abilities for levels 6, 8 and 17.
Overall, scarceness of cleric domains and rogue subclasses are probably the biggest let-down of the PHB, to the point that I'm already wondering if it's worth waiting for the summer 2015 PHB update. There's a small chance they'll make it 350 pages, like they decided to do with the MM.