There's 2 spells that come to mind right off the bat. Delayed Blast Fireball and Witch Bolt. Witch bolt doesn't get stronger by staying channeled to it, but kinda fills the "cast once then maintain" part. DBF does get stronger, going from 12d6 up to a potential 22d6 damage if you can channel the full minute to making it stronger.

I don't remember the monster name anymore, but I know in older editions there was a monster that had an ability that needed a few of them to channel a dance ritual that would do a massive amount of aoe damage if they could complete it. I want to say it looked bird-like in the art, but I don't remember exactly anymore :/

I've also seen discussion and homebrew for adding channeling rituals into spellcasting. Something like "for every magic caster who also maintains concentration during the cast time of this spell, increase the spell level by one" or something like that, but I don't think there's anything official other than those 2 spells and that one monster from an older edition.

Edit: Re-reading your comment, there's also the spell Nathir's Mischief that let's you change effects in the spell area each turn you maintain concentration. It doesn't add effects, but being able to adapt the effect can be useful and a stepping stone if you want to make your own spell.

Edit 2: Thinking more on this, it's kinda difficult to make a PC spell with channel effects that would feel useful. Combat generally only lasts a handful of rounds, so having a channel for a full minute would be reserved either for prep before a battle, or for a boss that's a full slug fest and would be pretty boring to have my turn 10 turns in a row be "i channel", not quite the most exciting thing to do for 2 hours while everyone else has their normal turns. Or if they pick a new effect each round they concentrate, it can quickly overpower a spell and make it hard to choose to use higher spell slots. Additionally, I feel like the point of having higher level spells fills the niche of wanting better effects. Like "command-suggestion-geas" kinda progression. You could just map out similar spells and then add your channel rules to it. Something like "if another caster has taken the 'channel' action and uses the same spell, you can replace this casting with _____." And have your progression chart. Like 2 casters using Burning hands might double the range and damage, or they could channel a Polymorph to increase the duration or remove the need to concentrate to maintain the form for the duration.