What is something good to build around?
That's a bit vague. You're talking thousands and thousands of cards from 8th edition to current that are all legal in modern. Narrow it down for us a bit:
Do you want a competitive combo for tournament play? {Kiki Jiki, Mirror Breaker} + {Restoration Angel} and cards like {Pestermite} or {Village Bell-Ringer} is seeing top tournament rankings. Another solid one is {Melira, Sylvok Outcast} + persist creatures like {Kitchen Finks} and {Murderous Redcap} with a sacrifice outlet.
Are you looking for mana combos? There are a ton of em!
Are you looking for infinite combos? Mill? Damage? Tell us what you want to accomplish!
Personal favorites are:
Bloodcrank: {bloodchief ascension} + {mindcrank}
No more spells: {curse of exhaustion} + {Erayo, Soratami Ascendant} or {knowledge pool}
Doubling Jace: {doubling season} + {Jace, Architect of Thought} (use ultimate to grab a new jace and do it again, add tamiyo for real silliness.)
Somehow I keep leaning towards artifact decks, Esper usually. I will see about bloodcrank though, anything else ridiculous while we are there?
There is a shards era combo for infinite turns with that artifact that gives extra turns. It's from back when i very first started so idk much about it.
{Thopter Assembly} and {Time Sieve}
Sweet! Can also use {grand architect} and {master transmuter} to speed it out, tutors like {fabricate}, etc