Basic rules-additions for spellcasting

The RPG systems I'm at more than slightly familiar with are D&D and DSA/TDE and as such the following is going to be orientated along with those two systems.

While D&D uses spell slots and features some really useful spells "for free" TDE works with Astral-Energy and specific cost for spells. That aside D&D does not require a test for successful casting while TDE does so. Basically, spellcasting in TDE is a spontaneously usable skill, with specific checks for each spell and in D&D it is a matter of planning ahead, preparing the mightier spells ahead of time. Both systems have "cantrips", free spells but D&D has quite useful and powerful ones as opposed to TDE where all cantrips are more or less only harmless utilities, lighting candle stuff.

Both systems, in-game, require components for a casting, those might be gestures, an incantation and or physical components of either consumable or unconsumable nature. In older editions of TDE, the incantations and gestures were described as part of the rules, players and DMs were advised to at least ask for the incantation to be cited in-game. This is no longer true and in my opinion this has reduced the fun and the immersion during the game.

I suggest the following additions to your house rules:

1. make a ruling about the nessesary use of a focus for each spell. - no focus needed, any focus or a specific focus, like "dagger size weapon focus only" or "not with a staff" etc.
2. make a ruling about how a focus changes that spell. No changes? Longer duration? Less actions to cast? Better range? etc.
3. make a ruling about how a missing component/focus affects the spell. Impossible without? Aditional actions needed? etc.
4. think about implementing a casting-DC, either general or per spell, if you play D&D.
   I tried DC13 + Spell-level or Spellslot-level (the higher one) as a DC with good success, the appropriate proficency bonus applies.

Personally, I have seen groups who, playing D&D, use a specific DC per spellslot and use the proficiency to modify. But I have also seen D&D groups having a DC and a correlating proficiency per spell. So I guess anything is possible there.