CapnZapp
Legend
Regarding the general dissatisfaction with the Concentration rules. It has several impacts, but most notably (in my opinion) the way it gates out all the second-rate spells.

Let's use Treeantmonk's excellent Guide to Wizards 5e as a starting point. (Please don't argue about his spell ratings here. If you disagree, feel free to view my suggestions as examples only)
http://www.enworld.org/forum/showthread.php?450158-Treantmonk-s-Guide-to-Wizards-5e
Let's construct a few items that satisfy the above wishlist: specifically the one where only below-average spells can be combined (spells TAM rate orange or red, but not brown or better than orange). We're especially interested in spells rated poorly specifically because they require concentration. We definitely focus on combat spells, since outside of combat concentration is MUCH less of a drawback.
RING OF CONCENTRATION
While wearing a ring of concentration, you may cast and concentrate on any Inscribed spell, even if you're already concentrating on an Inscribed spell. Any given ring of concentration has three or more spells inscribed. A ring of concentration only allows you to concentrate on two castings simultaneously and the spells must be inscribed spells.
Jagged Ring of Concentration
A Ring of Concentration with the spells Witch Bolt, Crown of Madness, and Bestow Curse inscribed.
Saintly Ring of Concentration
A Ring of Concentration with the spells Protection from Evil and Good, Ray of Enfeeblement, and Slow inscribed.
Profane Ring of Concentration
A Ring of Concentration with the spells Protection from Evil and Good, Darkness, and Vampiric Touch inscribed.
Silver Ring of Concentration
A Ring of Concentration with the spells Gust of Wind, Phantasmal Force, and Enlarge/Reduce inscribed.
Electrum Ring of Concentration
A Ring of Concentration with the spells Magic Weapon, Cloud of Daggers, and Haste inscribed.
Gold Ring of Concentration
A Ring of Concentration with the spells Hold Person, Stinking Cloud, and Fly inscribed.
So if I found one jagged and one electrum ring of concentration and wore both, I could cast Crown of Madness even if I'm already concentrating on Cloud of Daggers. Or I could cast Magic Weapon twice. You get the idea.
I would still have to spend spell slots as normal. I would still have to make concentration checks as normal (one for each spell)
The only difference would be that I no longer dismiss casting Bestow Curse out of hand, since I now can combine that with at least some other spells /Zapp
Yes, let's follow up with some specific rules languageA much more interesting solution would be:
Step 1: Identify at least one, preferably dozens, spell(s) that optimization guides rate poorly and that you never see used at your table.
Step 2: Add "rings of concentration" and similar magic that allows you to use those spells (and only those spells) even if you're already concentrating.
Specific rules language to follow; this just the general idea.
The ideal would to allow you to combine one buff that is currently rated red or purple, with another buff also currently rated red or purple.
You should not expect to combine any buff with one currently rated blue or gold, since that would be a direct power-up.

Let's use Treeantmonk's excellent Guide to Wizards 5e as a starting point. (Please don't argue about his spell ratings here. If you disagree, feel free to view my suggestions as examples only)
http://www.enworld.org/forum/showthread.php?450158-Treantmonk-s-Guide-to-Wizards-5e
Let's construct a few items that satisfy the above wishlist: specifically the one where only below-average spells can be combined (spells TAM rate orange or red, but not brown or better than orange). We're especially interested in spells rated poorly specifically because they require concentration. We definitely focus on combat spells, since outside of combat concentration is MUCH less of a drawback.
RING OF CONCENTRATION
While wearing a ring of concentration, you may cast and concentrate on any Inscribed spell, even if you're already concentrating on an Inscribed spell. Any given ring of concentration has three or more spells inscribed. A ring of concentration only allows you to concentrate on two castings simultaneously and the spells must be inscribed spells.
Jagged Ring of Concentration
A Ring of Concentration with the spells Witch Bolt, Crown of Madness, and Bestow Curse inscribed.
Saintly Ring of Concentration
A Ring of Concentration with the spells Protection from Evil and Good, Ray of Enfeeblement, and Slow inscribed.
Profane Ring of Concentration
A Ring of Concentration with the spells Protection from Evil and Good, Darkness, and Vampiric Touch inscribed.
Silver Ring of Concentration
A Ring of Concentration with the spells Gust of Wind, Phantasmal Force, and Enlarge/Reduce inscribed.
Electrum Ring of Concentration
A Ring of Concentration with the spells Magic Weapon, Cloud of Daggers, and Haste inscribed.
Gold Ring of Concentration
A Ring of Concentration with the spells Hold Person, Stinking Cloud, and Fly inscribed.
So if I found one jagged and one electrum ring of concentration and wore both, I could cast Crown of Madness even if I'm already concentrating on Cloud of Daggers. Or I could cast Magic Weapon twice. You get the idea.
I would still have to spend spell slots as normal. I would still have to make concentration checks as normal (one for each spell)
The only difference would be that I no longer dismiss casting Bestow Curse out of hand, since I now can combine that with at least some other spells /Zapp